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"Rare possible case? You hit Vecna with Magicbane and get confused, then he reads a scroll of teleportation, sending him some where ...in the entire game."
Although he is the second highest level spellcaster one will see in SLASH'EM, in general Vecna is nowhere near as tough as one might think. He is somewhat slow, and has only one spellcasting attack. His main weakness, though, comes from the design of his level; it has no up staircase at all. Being covetous, Vecna will teleport to meet you. Since there is no up staircase, he will not warp away like most covetous monsters, but will stay next to you, even as his hit points drop.
Elbereth, which he respects, is a must when fighting him, since his high base level means that his psi bolt spells will do immense amounts of damage. Additionally, one does need a +4 weapon to damage him at all. However, with a permanent Elbereth and a +4 weapon, he is normally quite easy to defeat, in many ways less trouble than the demiliches that surround him who will try to run away when you hit them. Also be aware that he will not attack until you come within sight of him, so it is very possible to choose when you fight him.
An exception to this behavior, is when he is generated with a wand of teleportation. In that case, every time his hit points drop too low, he will zap himself and teleport somewhere in the level. He will not warp back to you until he has healed. Considering that a wand can have more then ten charges, this greatly increases the difficulty of the fight. Depending on your situation, you'll have to rethink whether you should come back later.
The Chaotic Quest itself can be a difficult branch though, and should not be underestimated. It is advisable to bring an escape item or two, just in case something goes wrong with the general strategy. For example, if Vecna has a wand of teleportation or if a minotaur is summoned right next to you.
Vecna first appeared as a monster in NetHack Plus, a precursor to SLASH'EM, as "Master Vecna", the Undead Slayer quest nemesis. His stats were suited to the function; his base level was 25, rather than 49, and his melee attack did only 3d6 damage. However, it was already necessary to have a +4 weapon in order to damage him.
In SLASH 6, which used code from NetHack Plus, Vecna became a unique "boss" lich presiding over his own lich-populated special level, similar to the Rat King, Grund the Orc King, etc. Unlike in SLASH'EM, however, killing Vecna could produce either the Hand of Vecna or the Eye of Vecna, an artifact eyeball, with a 50% chance of each.