Walks around the room, avoiding Isaac. Upon death, spawns several flies, which can be Flies, Attack Flies, and/or Pooters. Can self-detonate and launch four projectiles in the cardinal directions while still releasing flies.
Chases Isaac, dealing a full heart of contact damage. Upon taking enough damage, it collapses into a pile of goo, which can be killed. Will regenerate after a short period of time if not killed quickly.
Slowly walks towards Isaac. Occasionally leaps off-screen and attempts to land on Isaac, leaving a puddle of Red Creep and shooting 6 blood shots in all directions where it lands. Has a chance to turn into a Blubber or Half Sack upon death.
Floats toward Isaac and occasionally fires a blood shot at him, predicting his movements.
Lil Portals are entities that only appear in Mausoleum and Gehenna. 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:
Very erratically hops toward Isaac in large groups, dealing contact damage.
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.
A special version of the Portal (dubbed the "Entrance Portal" to avoid confusion) serves as the entrance to The Void. 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.
As a result, a permanently charmed Death's Head can spawn from charmed Portals on Chapter 3 and remain permanently invulnerable. This is the only known method of charming an invulnerable enemy.
If a portal is spawned in either Chest or the Dark Room via Debug Console, the Portal will despawn a few seconds before spawning any monster.
If Portals aren't unlocked, certain enemies will spawn in their rooms instead, depending on the floor the room is in. For instance, Frowning Gapers spawn as Portal placeholders on Chapter 1 floors, whereas Vises spawn as Portal placeholders on Chapter 4 floors.
The room darkening as the player approaches the portal.
The Entrance Portal next to the Chest after defeating Isaac. Notice the distortion effect on the chest.
A portal's idle animation, as found in the game files.
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 Cathedral. This was later removed.
As of Booster Pack #5, 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.