[Memnoch]

Here's some info that Glorick the Half Orc posted here about damage output by kensai comparing dual wield with two handed.

The whole purpose of a Kensai is to deal out more damage than you take. A LOT more. Your THACO is much better than a regular fighter, and the Kai ability allows you to do MAXIMUM damage with a weapon for the duration of kai (about 6 seconds real time). Now, with dual wield and all your profs in Long Sword using Kai: At level 8, 3 attacks per round: 1d8 + 7 (str) = 15 (max) * 3 (attack per round) = 45 HPs of damage per round for 2 (3?) rounds. No other character can deal out anywhere near this damage. With a two-handed sword and two-handed wield, a level 8 Kensai will do: (1d10 + 7) * 2 = 34 HPs of damage per round. Now the most powerful weapons in the game are two-handed swords, and some of them have amazing speed factors. At the end of the game, your kensai could be doing: ((1d12) + 10 (girdle) + 4 (sword bonus)) * 4 = 104 HPs of damage per round!!!! With a thaco of -11 (in other words, you automatically hit AC -10), you will kill almost anything solo in two rounds of kai.

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Memnoch - Custodian of the Order of the Holy Flame