Portal

Portals are enigmatic entities that appear on many floors/ only in. They continuously spawn enemies until they are destroyed or spawn a certain number of enemies.

Portals spawn different enemies depending on the floor they appear on.

Lil Portal
Lil Portals are entities that only appear in and. They continuously spawn fly and spider enemies until they are destroyed. There can be a max of 4 spawned monsters alive at once. Monsters they can spawn include:

Lil Portal (Flies Only)
There is a variant of the Lil Portal with the ID 306.1.1 that looks and behaves identically to regular Lil Portals but will only spawn Fly related monsters from its regular pool.

Entrance Portal
A special version of the Portal (dubbed the "Entrance Portal" to avoid confusion) serves as the entrance to. It is found in, but can also be found after defeating Mom (5% chance and only if Isaac has 10+ tear damage or 20+ items), Mom's Heart/It Lives (10% chance), Isaac/Satan (15% chance), ???/The Lamb (20% chance), or Mega Satan (50% chance). The Entrance Portal room in ??? gets darker the closer the player is to the portal.

Trivia

 * Before v1.06.0185, portals continually spawned enemies until they were defeated.
 * Portals were able to spawn Bosses such as The Haunt on floors such as the . This was later removed.
 * As of Booster Pack, Portals will now leave a Purple "blood" splatter when killed by the player. This effect is only for aesthetic purposes. Formerly, they left the generic Red blood splatter.
 * Lil Portals resemble Warp Zones from Super Meat Boy.