Dota 2/Purist Thunderwrath the Omniknight

Omniknight is a melee strength hero that is usually played as a support or semi-carry. He possesses some of the best protective spells in the game, keeping himself and his team alive during the toughest fights. His Purification is not just a good damage spell, it also heals the targeted ally, potentially saving the life of a surrounded teammate. His Degen Aura, even though it has a smaller radius than other auras, will dramatically slow enemies that enter its range, making chasing down enemies a cinch. Omniknight is perhaps best known for his 2 defensive spells, which are Repel and Guardian Angel. Repel grants an ally magic immunitym while Guardian Angel gives nearby allies complete invulnerability to physical attacks. Omniknight is certainly a priority target during teamfights. If he lives, the rest of his team wouldn't be dieing very easily.

Purification
Purification heals the target by a set amount while doing damage to nearby enemy units. The damage dealt is pure damage, meaning that it ignores armor and magic resistance. A 360 hp heal is good enough by itself, but being able to do damage while healing makes this an excellent spell to use during teamfights and pushes.
 * Ability: Target Unit
 * Affects: Allied Heroes
 * Damage Type: Pure
 * Heal/Damage: 90 / 180 / 270 / 360
 * Radius: 225
 * Mana Cost: 100 / 120 / 140 / 160
 * Cooldown: 10

Repel
Repel temporarily grants magic immunity to the target. Interestingly it can be casted on allies and enemies, though there really is no purpose for casting it on enemies. Having this spell is like having a free BKB, except that unlike the BKB, Repel can be purged. Magic immunity is still always a welcome buff, and can let your important teammates like the team carry to shrug off enemy spells.
 * Ability: Target Unit
 * Affects: Heroes
 * Duration: 6 / 8 / 10 / 12
 * Mana Cost: 50
 * Cooldown: 14

Degen Aura
Degen Aura slows the movement speed and attack speed of all nearby enemies. While it has a smaller radius than most other auras, its slow is quite powerful, especially for a passive skill. Generally you want to keep up with fleeing enemies to slow them down for your teammates, but it is not a good idea to run around in the middle of a teamfight, since you'll likely get focused down.
 * Ability: Passive, Aura
 * Affects: Enemies
 * Move/Attack Slow: 7% / 14% / 21% / 28%
 * Radius: 315

Guardian Angel
Guardian Angel makes all nearby allies to become impervious to physical attacks as well as greatly increase their health regeneration. Naturally it is a very powerful defensive buff that can let your team survive repeated poundings from strong physical damage-dealers like carries. Combined with Repel, it can effectively make one ally completely invincible.
 * Ability: No Target
 * Duration: 5 / 6 / 7
 * Health Regen: 25
 * Radius: 600
 * Mana Cost: 125 / 175 / 250
 * Cooldown: 150

Aghanim's Scepter can be bought by Omniknight and will make Guardian Angel global and affect buildings.