Special Monster Mechanics

Dark Vessels are the core and namegiver of the so-called Transformers monster set, which is one of the best monster types to find when pushing. They spawn in packs of 3 or 7, will stay passive until you engage them and then take some time to transform into Unholy Thralls (you need to DoT them up or stay close for that to happen).

When found in large density, they can add a lot of extra progression (0.33% extra per each transformed mob) and even produce one-floor-rifts. Be careful with killing them too early as they are very very squishy!

These demons can be found in the Bogans Maggots monster set. They summon a Hell Portal that spawns several Armored Destroyers and Oppressors as well as a Mallet Lord. They are also really dangerous as they shoot projectiles that inflict you with a 50% Armor debuff.

You can find them with the Illusionist Affix if they spawn as minions with a Yellow Terror Demon. This Elite pack is by far the best when you encounter it in a push, as they can spawn 100% progress within just a few minutes. But beware: fighting 50 Mallet Lords and Oppressors at once isn't easy and requires very careful gameplay!

Grotesques, Horrors and Abominations are great monsters that follow you easily and stack up well. Upon dying, they explode and spawn a few Corpse Worms. You won't get much extra progression here but because you refuel the pull with fresh enemies all the time as you finish off more Grotesques, you can use those worms to keep your DPS high through Area Damage and finish tanky trash and even Elites much more easily.

Fallen Shamans exist in rather bad monster sets (Red Fallens and Blazing Guardians), however they give you the advantage of providing endless Area Damage and easy to reach on-kill effects infinitely by reviving small Fallens.

These revived enemies won't give any progression, but can be helpful to give you the density needed to kill some other of the bigger enemies or Elites. The Rift Guardian Ember starts spawning Fallen Shamans one by one below 66% HP.

These big Maggot Broods spawn small Maggots that can fuel Area Damage and on-kill effects (such as giving stacks for Archon Wizards).

Maggots can only be spawned when the previous wave has been killed (or if only partially killed, it will be filled up to the maximum again). You can find Maggot Broods as Champion Packs and as Yellow with Maggot Broods as minions. In both cases, they can roll the Illusionist Affix.

These can only be found as Elite packs in Greater Rifts, but can be very useful. They spawn 2*5 small rats in waves, which adds a lot of extra value for Area Damage (especially since these can be stacked very tightly).

They are not too uncommon to find as Elite packs, which can either be as a Champion Pack or a Yellow with Rat Caller minions. In both cases, rolling the Illusionist Affix is possible and can give you hundreds or even thousands of rats after some minutes, which is very powerful for extra progression and AoE damage.

Found in the Bogans Maggots monster set, these bats can take small Boggits and shape them into fullly-fledged Tusked Bogans, which then can proceed to spawn more Boggits. They can only do this procedure twice.

Bonus: It's possible to find Shrieking Terrors as Elite packs with Illusionist affix, which gives you effectively an infinite supply of Tusked Bogans because these Illusions can spawn them over and over.

Tomb Guardians can be found in a variety of good monster sets like Lacuni/Phasebeasts or Ghouls/Summoners. They summon waves of Skeletons that grant rift progress regularly.

You can find them with the Illusionist Affix if they spawn as minions of a Yellow Unburied. This Elite pack can give you hundreds of Skeletons within minutes, which is very powerful to advance your rift progress in a push.

Wretched Mothers are found in two good monster sets (Zombies Grotesques and Lacunis Phasebeasts) and are really useful as they spit out extra zombies for you to kill. When you walk close to them, they will vomit 2 zombies, which can be repeated up to 5 times every 15-20 seconds. When you encounter a pack of these while pushing, try to not kill them too early.

It is possible to find Dust Retchers as Elite Packs, which spawn 3 zombies per wave instead of 2. You can also encounter Elite Packs with a Yellow Zombie and Wretched Mother minions that can be Illusionist, resulting in endless waves of zombies.

Blood Clan Sorcerers and Occultists are extremely bad monsters. They are ranged enemies with one of the worst collision box in the game and they regularly cast a protective shield on a random nearby monster which makes those almost impossible to kill until it's over.

Corrupted Angels are very dangerous enemies. They charge at you from far away, knock you back when you have no Crowd Control Immunity and are just generally not great to fight.

On top of it, they have a certain hammer smash attack which deals multiple times more damage than normal and typically oneshots anyone, sometimes even including tanky support builds. Always be careful with these, especially when your cheat-death is on cooldown!

Fallen Lunatics are self-sacrificing enemies that run towards you and then explode for very high damage, typically oneshotting most players. You won't get any on-kill effects, XP or progression for your rift. They are also spawned by the Rift Guardian Ember from 95% to 66% HP and then below 25% HP. Make sure to stay away from these!

Morlu Legionnaires are generally some of the worst enemies to fight, but sometimes you have no choice. What's special about them is that they have a healing ability that kicks in once they reach 1 HP. This sets them back to 5% HP, removes all DoTs and debuffs and puts them in a stone form for a few seconds.

If you can't finish them off (which is quite likely in some pushing scenarios), they will heal back to 75% HP. This can repeat infinitely. Be especially wary about this mechanic when you find this monster type as an Elite pack! Frailty and The Executioner are extremely potent to execute these monsters before they can heal.

Vile Swarms are one of the best monster types in the game due to the following factors:

1. they come in large packs
2. they are very fast (for kiting)
3. they give good progression for their HP
4. they do not have a collision box

Especially the last point is very important here: you can stack them up infinitely tightly just by kiting them around in circles, and make it very easy to obliterate them with very few attacks due to insane Area Damage scaling. If you find many of these in the same place, every single Greater Rift map turns out to be great because all you need to do is blast them within a few seconds and move on.

It's worth noting that these enemies are also considered flying monsters and move over small obstacles or holes in the ground or even the edge of the map in some cases (e.g. Realmwalker layout for instance).