I'm thinking of introducing an Erlking a long-term adversary (and top lieutenant to the BBEG) in my current campaign.
Glancing through the statblock, I see:
Melee +3 cold iron keen longsword +20/+20/+15 (1d8+12/17–20 plus bleed)Ranged +3 ironwood longbow +20/+20/+15 (1d8+3/×3 plus bleed)
Special Attacks bleed (1d6), favored enemy (all humanoids +10), ironwood mastery
So that means he's getting +30 to hit against most PCs (who are humans, elves, etc.)? That's really, really dangerous.
But not against summons, companions/eidolons/phantoms, planar ally buddies, any Outsider(Native) PCs, PCs possessing non-humanoid bodies, constructs they've made, and so on. He is CR 18, though, and +30/+30/+25 is a lot better than the melee on a comparable dragon. Miss chance is the PCs' friend, along with sneaky tactics to avoid presenting humanoid targets.