Dota 2/Rylai the Crystal Maiden

Crystal Maiden is a ranged intelligence hero that is almost always played as a dedicated support. She has a very helpful aura that covers the entire map, giving her allies mana regeneration. She is also a great disabler with her Crystal Nova and Frostbite. Her ultimate, Freezing Field, can hinder any hero's escape as her teammates catch up. Crystal Maiden, being a support intelligence hero, is very frail. Get caught out of position, and she will go down within seconds.

Crystal Nova
Crystal Nova lets you cast a damaging burst of ice that slows enemy movement speed and attack speed. A useful slow, it can slow a bunch of enemies at once. The movement speed slow inhibits an enemy hero's escape, while in the late game the attack speed slow helps nullify the enemy carry's high damage output.
 * Ability: Target Point
 * Damage Type: Magical
 * Damage: 100/150/200/250
 * Duration: 3.5/4/4.5/5
 * Radius: 400
 * Movement Slow: 30%
 * Attack Slow: 20%
 * Mana Cost: 100/120/140/160
 * Cooldown: 15

Frostbite
Frostbite encases an enemy unit in ice, preventing them from moving or attacking but still let's them use abilities other than teleportation abilities. It is a very useful spell for holding a mobile hero like Storm Spirit in place, as well as disabling heroes that rely on their normal attacks a lot. Learn which target will be disabled the worse with Frostbite. Disabling a Lina won't be very helpful to your team since she can still use her high damage spells, but disabling Anti-Mage will make him almost useless to his team.
 * Ability: Target Unit
 * Damage Type: Magical
 * Damage: 70
 * Duration: 1.5/2/2.5/3
 * Mana Cost: 115/125/140/150
 * Cooldown: 10

Arcane Aura
Arcane Aura is a passive skill that will grant a boost in mana regeneration for every ally on the map. The effect is doubled for yourself. At it's highest level, it grants about 2 bonus mana regen per second, which is about 25 levels of intelligence. In comparison, the Ring of Bassilius only grants 0.65 bonus mana regeneration, and the aura is stackable with other items that grant mana regen. A very helpful ability especially if you have any mana-intensive heroes on your team.
 * Ability: Passive, Aura
 * Mana Regen: 0.5/1/1.5/2

Freezing Field
Freezing Field makes ice shards rain down randomly around Crystal Maiden. Every enemy in the area of effect will be slowed by 30% as well as take damage from the ice shards. A very powerful ability often overlooked because of Crystal Maiden's commonly taken support role, it has a very large radius for a slow, and the damage is just icing on the cake. Unfortunately, the ability is channeled, meaning that it can be canceled with a simple mini-stun, and leaving you vulnerable to being focused down by enemies.
 * Ability: No Target, Channeled
 * Damage: Magical
 * Radius: 685
 * Explosion Radius: 215
 * Movement Slow: 30%
 * Damage: 105/170/250
 * Scepter Damage: 170/250/310
 * Mana Cost: 200/400/600
 * Cooldown: 150/120/90

Aghanim's Scepter can be bought by Crystal Maiden and will increase the damage of Freezing Field.

Suggested Skill Builds
There are two most common ways of skilling Crystal Maiden. One thing is common between these two, in that her ultimate, Freezing Field, is usually skipped at level 6 and only skilled at around level 10. It is usually left at level 1 and sometimes not skilled at all. This is because Crystal Maiden is quite squishy, and performing a Freezing Field leaves you vulnerable to attacks. It also costs a lot of mana, and might leave you with too little to cast your disables. The slow is constant throughout all levels and is commonly considered to be more important than the damage, since you aren't supposed to be a damage-dealer anyway.

The Aggresive/Roamer Build
This build takes advantage of Crystal Maiden's excellent early-game power by focusing on her disables. Get Crystal Nova at level 1, Frostbite at level 2 and proceed to alternate between Crystal Nova and Frostbite until both are maxed. Get an early level of Arcane Aura to give you enough regeneration to keep casting spells, and max it last. Freezing Field should be skipped at level 6 and ideally skilled at level 10 or 11 when you have a larger mana pool.

The Defensive Build
This build is not as popular as the aggresive build, but it focuses on improving your Arcane Aura, which benefits all allies globally. It can be useful if your team has multiple mana-intensive heroes. Get one level of Crystal Nova and Frostbite so that you can use them when needed, then max out Arcane Aura as soon as you can. After that, max Crystal Nova and Frostbite. Like the last build, you should skip your ultimate at level 6 and get it later.

Suggested Item Build
This item build assumes you're playing as a support. You won't be getting a lot of gold, so you'll have to get by with what you have saved. Crystal Maiden can be considered as a "ward bitch", though it's meant as a compliment, as she can be the designated ward buyer. This will leave you with even less gold to spend for yourself, so getting cost-eeficient items is a must.

Starting items
Get the Animal Courier or the Observer Wards if noone else has bought it. Use your remaining gold to buy a combination of Iron Branches, Healing Salves and Tangos. If you think you need more survivability, get a Gauntlet of Strength as well.

Early-game items
Try to get a Magic Stick and maybe a Magic Wand to give you some survivability against ganks. Go for a Flying Courier if your teammates don't, and be sure to replenish your team's Observer Wards constantly. If you manage to get enough gold you can go for a Boots of Travel.

Mid-game/Core items
Observer Wards are still a priority at this time, as well as other utility items like Smoke of Deceit if your team wants to smoke gank or Dust of Appearance and Sentry Wards to counter invisibility heroes. You can also consider getting a Bracer for some cheap extra survivability. For your boot upgrade, Power Treads gives you some much needed survivability. Tranquil Boots is also a good choice since it is quite cheap and gives you a good heal, while you can diassemble its components to create a Mekansm later on. Arcane Boots is a decent choice, but it's a bit redundant since you already have Arcane Aura.

Go for a Mekansm or a Pipe of Insight, as they're great support items for use in teamfights, but don't get both since you'll be a target of the enemy team, and if you die your team loses a lot of survivability that your Mekansm or Pipe provides.

Luxury/Situational items
If your team has the upper hand and you find yourself having a good amount of gold, you might purchase a Gem of Sight if there are invisible heroes on the enemy team, but be careful since it drops on death and you;re quite squishy. A Force Staff or a Eul's Scepter can also be useful, the Force Staff to give you or your team an escape mechanism while the Eul's can let you or an ally escape harm temporarily, as well as being a good disable.

If you have a lot of money leftover from getting utility items, go for an Orchid Malevolence or a Scythe of Vyse for excellent disables. You can also go for a Black King Bar which will allow you to pull off a worthwhile Freezing Field without getting interrupted by a mini-stun.

Laning/Early-game
You have two choices, babysit the team carry or go roaming. Supporting in lane is a more popular choice. Since you're the lane support, you should let your lane partner take all the last hits, while you can focus on denying and harassing the enemy. Crystal Nova is very handy for harassment as it can hit multiple enemies, which will usually force them back, but you might push the lane. If the enemy attempts to gank, Crystal Nova can also ward them off. Your Frostbite is very handy if an enemy hero overextends and dives your tower, as it can usually net you and your partner a kill. Be very careful with your positioning and avoid dieing pointlessly, as a wrong move will likely get you killed.

A riskier strategy is to go roaming, which can give your team an early advantage if executed properly. Your Crystal Nova and Frostbite combination is very effective early on, and the combined damage and disable makes them easy for your allies to finish off. Make sure to coordinate with your allies when you attempt to gank a lane, so that they will be in position when you initiate the gank. Be sure to stay active around the map when roaming, because you could end up too underfarmed and land your team at a disadvantage instead.

Mid-game/Teamfights
Around the mid game when towers start falling, do not stray too far from your teammates. You are still as fragile as ever, and you cannot survive if you were caught off-guard. Use your Crystal Nova during pushes as it can help clear creep waves. Remember to keep having wards on the map to get vital information about the enemy's movements. Your spells will do a little less relative damage, so you will rely on nearby allies to help you chip down enemy health.

During teamfights you should try to hit as many heroes with your Crystal Nova as you can, especially any carries or semi-carries, to slow their attack speed. Frostbite is also useful for pinning down any priority targets. Try to stay outside the teamfight but close enough that you can still cast spells, since you simply can't survive very well in the middle of a battle. As for Freezing Field, pulling one off successfully is difficult because you have to be in the middle of a bunch of enemy heroes and you're completely vulnerable while channeling, so using it at the wrong time usually results in the spell getting cancelled or getting yourself killed. It can be very effective if your team has used large teamfight spells like Black Hole that keeps enemies disabled for a short while, or around the end of a teamfight where your team has the upper edge and enemy heroes are frantically escaping.

Lategame
Again, stay close to your allies so you don't get melted within seconds when you accidentally encounter an enemy hero. Your spells and your normal attacks won't do nearly enough damage to matter at all, so all you need to do is focus on disabling heroes and survival. Be very careful not to get killed before you can contribute to your teammates, because if the enemy carry sets their sights on you, you won't be living very long. Try your best to land your spells, especially Frostbite, on an enemy carry, which will help your carry defeat theirs. Of course, you can also help your teammates take down other enemy heroes, but the carry remains as the biggest threat.

Skill synergies
Crystal Maiden doesn't have any complicated spell combos. The most basic is just to follow up Crystal Nova with a Frostbite. While the movement disable might cancel out the slow, it is still effective at pinning down your target while slowing another. Arcane Aura works well with all of your spells as it lets you use them more often without worrying about running out of mana. Freezing Field doesn't synergise particularly well with your other spells except Frostbite, since you don't have anything that will keep enemies from attacking you while you channel the spell.

Team synergies
Crystal Maiden synergizes very well with just about any hero with her good disables, but some carries like Juggernaut and Ursa can do a lot of damage and get some easy kills with her help. Heroes with strong but tricky to hit stins like Lechrac and Kunkka can land their stuns with the help of Frostbite. Any mana-hungry spellcasters will also appreciate your Arcane Aura. Heroes with good initiation or teamfight spells like Enigma and Magnus can help set up a worthwhile Freezing Field while diverting attention away from you with their more threatening spells.

Counters
Crystal Maiden is rarely ever a threat. Her teammates are usually the players you need to be concerned with, though she can help them do a lot of damage to you. Crystal Maiden's most obvious weakness is her low health, so any high burst of damage can quickly bring her down. Like other spellcasters, Crystal Maiden is also countered by heroes with Silence or mana-draining abilities, because her physical attacks simply aren't effective at all.