The Elder Scrolls IV: Oblivion/Oblivion Gates

Oblivion Gates are portals of some kind to Dagon's realm of Oblivion. It looks very evil, with lava (which you can swim through or use a Water Walking Spell on, although it rapidly damages your health), Daedric creatures, and even plants that fight back!

Oblivion gates are a staple in the Main Quest, and according to Martin, the portals shouldn't be possible. But since the Emperor died at the start of the game, and the Dragonsfires are dark, they can now be summoned. You can close them, of course, but it isn't easy. There are two reasons why you should make it a practice:
 * 1) Taking the Sigil Stone in the tower will close the gate, but you can keep the stone to make Enchantments. At the highest levels, the Transcendent Sigil Stones offer the best enchantments in the game. Transcendent Sigil Stones start appearing at level 17.
 * 2) Closing Oblivion gates increases your fame. This is helpful for infamous characters (although you could also just complete the Pilgram's Way).

Oblivion gates don't appear much until after the "Dagon Shrine" quest in the Main Quest. Then one will appear outside of each of the 7 cities, except for Bruma. You will have to shut each of these gates for the "Allies for Bruma" quest, but it is optional to do this quest anyway. If you do it, you will gain some levels and get some Sigil Stones. The Cheydinhal gate is tied to a quest called "The Waywrad Knight", in which you have to find the Count's son.

In the Realm of Oblivion, the plants can fight back. The Harrada Root can swipe at you for five points of damage. The spiddal stick plant casts a Drain Speed spell if you get too close. The effect can stack, so be careful. You have nothing to fear from the bloodgrass plant.