Oh ye of little faith
No it wasn't a typo!
I have indeed managed a backstab of 2406 (no typo!) on an unarmed Sarevok, although this was a critical hit it was not quite with the optimum equipment, so higher damage is possible.
This uses some (legitimate) cheese within the game, although it's still possible to do over 800 points of damage without using cheese.
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I'll do the non-cheese version first.
You need to get one HLA for your assassin (use any item) and then dual to cleric. You need a bard (for the improved bard song). You need a scroll of tensers transformation and a scroll of shapechange. You need 25 strength (quite easy to achieve with a cleric), the gauntlets of extraordinary specialization and you need to be wielding the Equalizer in your off hand (it doesn't matter what weapon you have in your on hand, use your fist!). You need to backstab an unarmed (or missile weapon armed) chaotic evil adversary. You need to cast aid, bless, shapechange to Iron Golem (from scroll) and tensers transformation (from scroll).
Then hide your Iron Golem in the shadows (!!!!) and backstab away! This assumes you get a critical hit:
For an Assassin->Cleric(24->27)
Assassin Bonus : +1
Iron Golem's Fist : +40
Improved Bard Song : +4
Cleric Aid : +1
Cleric Bless : +1
Tenser's Transformation : +2
GoESpecialisation : +2
Unarmed : +4
Maximum Total : 55
25 STR Damage Bonus : +14
Equaliser Damage Bonus : +6
Total Damage = [(55 * 7) + (14 + 6)] * 2 = 810dmg!
I think you could use Vhailor's helm and get your simulation to do the backstab - then you don't even use up the scrolls.
Note the cleric is only required for the easier path to 25 strength and the aid and bless. An solo assassin/mage with a strength potion would be able to do 350+ damage (700+ on a critical hit) on a regular basis.
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Cheese version
The version uses the fact that Improved Bard Songs stack, including ones from misleads. So you need a custom multiplayer party consisting of an assassin/cleric and 5 bards. The bards each cast mislead x5 (one does x6 with ring of wizardry) and all bards and all misleads sing.
The rest is the same as before.
For an Assassin->Cleric(24->27) with 5 max level bard helpers. 1 bard helper has a ring of wizardry, giving 5 bards and 26 misleads.
Assassin Bonus : +1
Iron Golem's Fist : +40
Improved Bard Song (+4*31) : +124
Cleric Aid : +1
Cleric Bless : +1
Tenser's Transformation : +2
GoESpecialisation : +2
Unarmed : +4
Maximum Total : 175
25 STR Damage Bonus : +14
Equaliser Damage Bonus : +6
Total Damage = [(175 * 7) + (14 + 6)] * 2 = 2490dmg!!
Misleads hang around for 1 round/level, so if you really wanted to you could rush a bit and add some extra cheesiness with some spell traps (scroll and/or staff) and pre-prepared symbols and get even more clones... or maybe 5 chain contingencies (cast earlier from scrolls) each with 3 x Mislead. Or maybe some project image scrolls? Or all of the above? :mischief:
You do have to wonder exactly what is going through your victims head while he/she/it is standing there being regaled by a chorus of 26 visible and 5 invisible bards...