Still in the box but...

1.Mummies
2.Ghosts/Spirits - not as enemies, they just wander around (oblivious to you) as if they were still alive, maybe in certain cases give you quests, or battle if you do something to piss them off.
3.Creatures made of stone that can mimic any stone objects. Since underground, plenty of pillars, totem pole type things made of stone. They can move around but usually let you pass by unnoticed until the opportune time to attack. You could start hitting anything thats made of stone for good measure but that might cause other problems...
4.A giant slug for the unexplained tunnels in parts of the city.

the underground poisoning has tarnished the land above, leaving mighty trees to poison and die, leaving their gnarled roots behind... angry, and hungry.

the very earth itself is moved to grieve from this abomination of mana. The mud grows foul, moving as with a life of its own, to drown those unlucky enough to be caught in its pools.

and in the deep places, left forever hidden from the sun, the dark things wait. Even they were not able to escape this plague, buried as they were beneath miles of stone and clay. Breathing the fire of the plague itself, the poisoned giants climb through the earth, to wreak havoc on the world above..

hehe, not only a couple ideas but i added some mood to boot. well, there would be a couple of monsters i would expect to find in an underground world.

but, on a side note, will the player be spending a great deal of time down here? From personal experience i have found that players' least favorite areas in games are caves, or other places where they're stuck for a great deal of time. Since you mention that this will take place in an underground abandoned city, please be sure to keep the scenery interesting.

Also, best of luck on your project, since you've made it this far.

I had an idea a while ago for a sort of hydra-inspired creature. The physical form doesn't really matter, the basic idea is it can separate into multiple copies of itself (and those copies can fuse back together), dividing up stats like health/strength/etc. among the copies. This can either be a sort of magical "projection" or actual physical separation.

One tactic it can use for example is to create a copy of itself behind you. The original occupies your front while the copy attacks your relatively undefended back.

Expanding on that idea from dobbs, and introducing something to alter tactics, something that splits into multiple copies of itself when hit, forcing players to ensure they kill them with 1 hit, or perhaps have to take special precautions to avoid the effect.

Monsters that mimic the player, objects, terrain, or NPCs. Maybe even monsters that have a shifting form, and varying abilities depending on the current form. They could have a certain form where its most advisable not to hit them in order to avoid splashback damage, and/or a form they arent ever in for long where you can kill them almost instantly.

Small flying monsters (think like bats), that are more of an annoyance than a threat - plenty of people are scared of bats. Dont annoy your players too much without good reason though.

How about monsters that the player doesnt even get to see properly. Think nasty sneaking, climbing critters, that set up ambushes, use ranged weapons, and disintegrate when killed, so the player never really finds out what they are. Also, in the vein of disintegrating when killed, monsters that arent particularly hard to kill, but which need to be killed carefully - they might dissolve into a pool of acid, explode, or turn to stone (trapping weapons), or emit loud, monster-attracting sounds when they die, forcing the player to take care in how they deal with them.

Dont forget good old fashioned environmental challenges. If you use those sufficiently, you wont need to rely on monsters as much. An abandoned underground city provides plenty of opportunity for this type of thing.

"Mana Plague" sounds like something ripe for mutation or at least giving animals/objects magical/enhanced powers that they don't usually have.

How about a venom-spitting Reptilian? Their poison would stick to you and slowly leech away your health until an antidote was found.

Or Screechers - bats that render you deaf for a time. When you recover you could have this horrendous ringing tone.

Big, fat sewer rats are always an option. Their eyes could glow red from the Mana. You could say that they have infrared sight, which would give them an advantage in dark maps/environments.

Intelligent horde behavior would be something interesting to simulate - bees/wasps or something.

Trap-laying super-spiders could be cool.

I don't know what the premise of your game is nor the direction of your gameplay but you could have each monster complement each other in the zone - say, if you're deaf from a Screecher, you can't hear the rats scurrying around. Or you could be routed into a spider trap using intelligent wasp-horde tactics.

Having enemies that explode when killed would add a really interesting dynamic to the combat. With a few of those mixed in, the player could take out a larger group of enemies by targeting the weaker, but explosive creatures. We're focusing more on smaller scale fights, but a pitched battle to hold off a horde of enemies is always fun every now and then.

Enemies complementing each other's strengths is also a good idea. I think this discussion has given us a much better premise to work with than the cliche zombie approach.