Personally those Shadow Dragons can be pretty annoying.

Bandits or Arachyura. Poison + doom or immobilizing two heroes at once is pretty annoying.

Changelings are pretty nasty. Thy're tough for a small monster, and can move you, and make you bleed and suffer fatigue. Sorcerers are heavy hitters with Sorcery, and I think Giant's multi-stun is worse than pincer attack.

On the rare occasion I play a hero and not the overlord: bandits. Bandits are horrifying to play against.

Goblins, just because they are all cowards !!

Only working from the base game but barghests tend to annoy me quite a bit.

Hard hitters, especially for the number of them the OL gets. Slightly tougher to kill from range.

And most of all, they can clog up 2-width paths. Line them up sideways and the heroes are up against a brick wall.

Chaos beasts. Espesh in act 2.

Bandits /sigh played well they are hard hitting and fast,

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Flesh Molders if hidden behind large units are also annoying

Goblin arches in some cases due to their movement can be a pain to catch!

I have not yet played as hero but I believe bandits is the worst. I have used them two times I think and both times they ripped the mage and the healer in pieces. They go down fast but they are many and if they are left alone they move fast and deals tons of damage in ACT 1.

Plus they can destroy loot. That's such a huge threat.

I can only go on what I have (Descent 2e, LotW, LoR and 1 H&M pack) and what my daughter's groan about the most, as I have never played a hero. When I pull out the Shadow Dragons (as I did last night for the Shadow Rune finale), they always get that wonderful look of dejection and defeat.

Harpies + imps. Without any blast ability on heroes + OL card "unseen wings" they can kill 1-2 heroes in one turn.

I'm particularly disgusted with the raven flock...