What's your surprisingly lethal monster?

[drquicksilver]

Sometimes I die to something which just leaves me staring at the screen in disbelief.

Twice I have lost promising (level 20+) @s to the humble energy vortex - did you know that it has a lightning bolt spell which does 5x as much damage as its breath weapon and probably 20x as much as its melee attack? The two @s both died when the vortex used the spell twice in a row and did 220+ damage in two rounds...

What has caught you by surprise?

[Estie]

Wiruin(?) the maelstorm. The first time ever I fought him I figured I would try out my (also first time ever) single combat spell. Lets just say it wasnt a brilliant idea.

[AceRimmer]

Surprisingly lethal? The humble Priest. Causing serious wounds turns out to cause serious damage (I believe it's 8d8) to a young descending adventurer. Priests have killed a few of my kind. Most recently, I lost a promising character when he was ganged up on by two priests (was this the one who found a Blade of Chaos of *Slay Orc* at 300ft?). It was one of those cases of attack, attack, attack, you have died, wtf?!?

I'd like to give an honorable mention to the colbran, though I'm not surprised any more when it kicks my butt.

[Sphara]

Early druid has a very nasty spell set for the depth it appears. It does not need many turns to kill a character that has no rFire, rElec and FA.
Priest is already mentioned and it is on par with a druid, when it decides to summon kin few times in a row.

[DavidMedley]

energy vortex - did you know that it has a lightning bolt spell which does 5x as much damage as its breath weapon and probably 20x as much as its melee attack? The two @s both died when the vortex used the spell twice in a row and did 220+ damage in two rounds...

My last character died this exact same way!!!

[Selkie]

Snakes are tougher than they used to be. In the old days I'd just walk through them. I've recently lost a few low level characters to poison just by being careless around reptiles.

[sffp]

Snakes are tougher than they used to be. In the old days I'd just walk through them. I've recently lost a few low level characters to poison just by being careless around reptiles.

What's the one that spits venom and blinds you? - the Black Mamba?

definitely on the list.

[sffp]

Umber Hulks for consistent confusion

Anything that can paralyze ghouls, homunculus

Basilisks, greater basilisks...

Wyverns are fast now in 4.2.1
Bats of Gorgoroth

Oh and necromancers with their nether bolts and medium wounds!

[Sky]

are you inviting us to dump half the monster list of angband here?

there's tons of "oh it's just a harmless X" monsters in angband that can kill you; but, i would go rimmer's pick, the Priest. Fails his spells most of the time, but occasionally they can absolutely nuke you.

[bughunter]

No. 1 The Louse

The. Louse.

--my first evar Mage

[Hounded]

Yup, the Louse. HT warrior level twenty-something... or maybe it was the hunger that got credit. Either way if the breeders hadn't stuffed the hall so effectively he'd not have been stuck there...

[Lemon]

My early adventurers die like flies. I lost a level 1 adventurer before to a green mold killing me with poison. I was definitely not expecting to die from full to poison, but that's what happens when you have 12 hp.

[Angdrim]

Not quite in the spirit of the question, but something off-screen breathing nether at me. Or Wiruin, who doesn't show up on ESP, so I'm always surprised when he kills me.

Or, even further from the spirit of the question, how about Osse? We're both trying to kill Morgoth, if I didn't know better I'd sure as hell be surprised that this guy is attacking me.

[Sphara]

Or Wiruin, who doesn't show up on ESP, so I'm always surprised when he kills me.

Heh. Wiruin sure is surprising the first time you meet him. I met him the first time as an uber-strong character who had cold immunity. Thought I could wipe him out. Going melee resulted into a screen filled with aether vortices and endless stream of sound breaths.
Along with the Q (greater ones and that nexus one) monsters, Wiruin is one of the few monsters I'm actually scared of, when running an endgame character.

[Sky]

i love louses, fleas, red worm masses, and to an extent, black oozes. Get yourself a nice long corridor, and you can jump to CL20 in no time.