Kas, V, is a unique monster that appears in EvilHack. He is the former right-hand servant of the powerful spellcaster Vecna, and is the second most-powerful vampire encountered in the game after Vlad the Impaler.
Kas is found within Vecna's Domain, residing in the stronghold that constitutes the branch's second level. He is generated asleep at the rightmost portion of the eastern room, standing on top of the down stair next to an iron safe and chest, and is always generated with his namesake sword.
In terms of base stats, Kas is slightly stronger than a vampire royal and "only" has 16 speed. Even so, he should absolutely not be taken lightly - on top of being fairly fast and hitting decently hard, the Sword of Kas can deal fearsome damage even if you poison resistance, and its various bonuses only further augment his already-strong pair of weapon attacks.
Kas the Bloody-Handed is derived from the Greyhawk setting of Dungeons & Dragons, where he was the former most trusted lieutenant of the despotic lich Vecna for many years. He ultimately turned traitor and brought about Vecna's temporary downfall, though details of his service and subsequent betrayal vary with the edition. The Sword of Kas was either created or procured for Kas by Vecna, and is consistently tied to Vecna's other relics - only the Sword and Vecna's hand and eye remained after their battle. Kas used the sword against his former master, and may have been goaded into doing so by the Sword itself.
How Kas became a vampire also varies depending on sources as to whether it was before or after the betrayal. In one such source, the battle between Kas and Vecna destroys the Rotting Tower they were in, leaving only the Sword and Vecna's hand and eye, and Kas is flung across the multiverse to another of Vecna's dwellings near the Negative Energy Plane, which changed Kas into a vampire - he then took on the moniker "Kas the Destroyer".
In EvilHack, Kas appears to still be loyal to Vecna, among other things implying that he was already a vampiric being beforehand.
Kas the Bloody-Handed was the most trusted lieutenant of the
despotic lich Vecna, and wielder of the Sword of Kas.
After many years of loyal service to Vecna, Kas eventually
betrayed his master. It is said that the sword itself whispered
to Kas, convincing him to slay his master and usurp his power.
- dat/vecna.des in EvilHack 0.7.1, line 196
- EvilHack commit 59f3575 - "New branch: Vecna's domain (part one)."
- EvilHack commit f0d1352 - "New branch: Vecna's domain (part two) - artifacts, Kas (unique monster)."
- EvilHack commit 919cd95 - "New branch: Vecna's domain (part three) - levels, minor tweaks."