Admittedly, I've been loathe to do this book because it has SO many great monsters. They are legitimately difficult to group and rank.
Here are the Underworld monsters in order from strongest AoE attack to weakest:
- The Ringer
- Kaih the Eradicator
- Lord Moltus
- The Judgement
- Greedy Dragon
- Worker Hulk (deals no AoE damage)
- Nishant's Pet (deals no AoE damage)
And here are those same monsters, now ranked by strongest single target attack:
- Kaih the Eradicator
- Worker Hulk
- The Judgement
- Greedy Dragon
- Lord Moltus
- The Ringer
- Nishant's Pet
It shouldn't be surprising to find that a monster that costs $20 is near the top of the list. Kaih the Eradicator deals the 2nd most damage on both of the lists. As a fire monster, he burns his enemies. He also has the rare ability to ignite enemies (which is a lot like burning and no one is immune to it). If you get both of those status effects on an enemy, after 3 turns, that enemy will have lost 85% of its life (from just those effects). The VIP Bulwark trait is also the best trait in the game. The only thing Kaih doesn't do is boost his own damage, but with his high base power and devastating status effects, he really doesn't need it!
We can't talk damage without talking about Barbatos. While his trait (immune to nightmares) is one of the worst traits, his abilities and stats more than make up for it. As you can see, unmodified, Barbatos is the absolute heaviest hitter of the Underworld monsters (at least on a single target). Getting into his skills, Blood Covenant gives energy regeneration, double damage, AND and extra turn (allowing you to immediately kill something). This comes at the cost of some of his life. Here's how it works: when your extra turn starts, in addition to all the awesome buffs, Barbatos will lose 20% of his CURRENT life. Then, he'll lose 20% of his new current life on his next turn. Barbatos' Master of Pain skill deals 90 dark damage to a single target but it costs him 30% of his MAX life (not current). Be careful as using this skill can kill him. So long as you pair Barbatos with a good denier (to prevent taking advantage of Barbatos' health) or a supporter who can heal him, Barbatos is an amazing monster finisher.
You'll notice that Greedy Dragon is the middle of both lists for base damage, but there's a couple good reasons you shouldn't keep him there. Firstly, his "The Magnificent" skill gives him damage boost and immediate extra turn. Most others have to wait another go 'round before taking advantage of a boost, but his is straight away. When that boost is factored in, Greedy Dragon deals some of the highest AoE and single target attacks of the Underworld (until they get their damage boost, but by then, Greedy Dragon may have already killed them!). Additionally, he burns, stuns, and steals life. His trait (energized) is probably the worst trait in my opinion.
Y'all, I love Rador. I wish I could rank him higher, though. First of all, he has the highest base power in the whole dang game BY FAR. He also ranks toward the top of each base damage list. Like many other fire monsters, he's immune to burning. His Turn them to Ashes ability deals damage to an enemy while giving himself double damage. If you can follow up that double damage with Flame Slam (70 special damage to a single target) and/or Mad Ride (45 fire AoE damage), you're going to kill something. He's also right in the middle of the pack with life and speed. He's really only beat out by some of these others because he has to wait a turn before making use of his double damage, but if you can pair him with a good supporter, he might be your go-to.
The Ringer is in this tier for one reason and one reason alone: his skill Death Knell. This skill deals 70 dark damage to ALL enemies and gives them bleeding. Other than Nox the Condemned, no other monster comes close to this level of AoE damage. Unfortunately, he can't boost his own damage, and his other skills are less than spectacular. If you can pair him with a supporter who can boost his damage, The Ringer can very well one-hit the entire enemy team.
Tephra deals more base damage than half the other Underworld monsters, but he also benefits from being to apply Sticky Lava to all enemies, causing them to receive +80% more damage from fire attacks. Sticky Lava (and other elemental weakness debuffs) are better than a damage boost so long as you continue to use that element. And while it's less than double damage, if you have more than one fire monster in play, they ALL benefit from it, whereas only your one monster gets a damage boost. So, depending on your team, Sticky Lava could end up being better than double damage!
Metalhëad is one of the more versatile monsters. He deals high damage, but he also does a fair bit of denial. He can stun all enemies, burn all enemies, and possess a target. Even though he may not be the greatest pure attacker (which is why he's in the third tier), his versatility is awesome.
Atlas seems like a monster searching for a purpose. He deals high damage (one skill deals 50 earth AoE damage) but then he has a tank skill (double life, receive all single target attacks), can give shields, and stuns. With a decent pairing, Atlas can deal serious damage, but his skills don't all seem lined up for that purpose.
Tyros is a simple, straight forward, classic attacker. He deals high damage and burns things. That's it. He has to be paired with someone who can boost his damage. His life is also not that high (he has the second lowest life of all Underworld monsters), meaning he'll have to be protected by a denier to really deal his damage.
I've always hated how The Judgement was spelled. At any rate, his base attacks put him consistently in the top half. Like most of this tier, The Judgement can't boost his own damage. But, given his Nature element, he can poison enemies and heal himself (abilities not often found on this list). Again, he needs to be paired with someone who can boost his damage.
Klectus' damage output is similar to the rest of this tier. The only thing that makes Klectus unique are his life steal abilities. If his damage is properly boosted, he'll be much harder to kill, as he'll have more life he'll be taking with each attack! And while I rarely mention special skills, his is worth noting as it gives a chance to instantly kill enemies. He also has the absolute lowest life of all Underworld monsters.
Worker Hulk is the OG damage dealer. In addition to damage, he stuns enemies and drains stamina. SP thankfully tweaked his stats not too long ago, but he's still a ridiculously slow brute. If you don't have speed runes on him, he'll rarely attack, making any power he has worthless. Being an earlier monster, he's just been outclassed at this point.
Ralbog is on the lower end of damage dealers. He causes burning, bleeding, and gives himself small shields. He's no where near as impressive as the mighty Balrog from whom he is inspired.
Erebus is in the bottom half of damage dealers in this book, but he's at the top of that half. He can also steal life, cause bleeding, possess, and give stamina leak. Erebus isn't a worthless monster, there's just so many better.
Lord Moltus is another early monster who has simply been outclassed at this point. He deals damage, he burns his enemies, and he can stun a single target. The other benefit of Lord Moltus (if you've been around long enough) is that he was available as part of the old loyalty puzzle. You'll find many older players using a ranked up version of him.
The only reason I'm even mentioning Vadamagma is that he's breedable. If you've got a Vadamagma up to level 130, his damage and other stats rival the best, so long as their lowly 100s!
Ouros is the ultimate tank in this game. Nishant's Pet and Lilysha are more deniers. And Demise is a supporter (and one of the best at that).