Vlad the Impaler
Vlad is the second fastest monster in the game, faster than a player with speed boots. His speed allows him to warp to you, hit you, bite you, and sometimes warp away again in a single turn before you get a chance to retaliate. Unlike other covetous monsters, if you are standing on the ladder yourself, he will warp away a few steps when hurt instead of staying next to the ladder. If you take the bait and follow, he may then escape down. This, combined with his high 130 HP, makes fighting him quite frustrating, especially for low-damage characters.
He hits hard, and often gets multiple hits for each of yours. Above all, he deals double damage during the midnight hour (0:00-0:59 server time) due to being undead, and he has all the powers of his subordinate vampires including drain life attacks. He is also immune to sleep, death rays, slowness, and polymorph. The quickest way to deal with him is to use a footrice corpse or a potion of paralysis. If neither of the two is available, you need decent AC and a well enchanted silver weapon to take him down quickly. Failing that, you can let him escape the tower into the main area of Gehennom, and stand on an upstair to fight him.
Be careful not to confuse Vlad: if he levelports by a confused scroll of teleportation, you get to look for him in his Tower, all of Gehennom above it, and the entire Dungeons of Doom, which is very annoying. This also makes Magicbane an impractical choice.
Prior to 3.6.1, Vlad used to be significantly slower, weaker and wouldn't teleport away. He was widely regarded as being quite a feeble enemy to be a mandatory fight at the stage of the game when he is faced; he was not significantly stronger than the vampire lords which appear as normal enemies.
Jokes abounded of fighting him with food rations, magic markers, thoroughly rusty thoroughly corroded tin openers (which could still rust back then) or orcish daggers, not moving every second turn while fighting him, or a variety of other absurd scenarios which would normally result in YASDs, and still easily defeating Vlad. The items used for such exploits had become known as Vladsbanes (or Vladbanes or Vladsmashers, in a mockery of the Banes) and on the occasions when such an item is used it is often so #named in tribute (see link below). The danger of confused levelport by Vlad even gave you a legitimate reason to use a Vladbane-type weapon instead of Magicbane.
A variety of patches had been written before 3.6.1 to address this issue by altering Vlad in some way. One of the better known ones was the aptly-named Vlad Balance patch. The Dudley's dungeon strip for 30 November 2006 (via Wayback Machine) featured several humorous attempts to remake Vlad; see also the comments for the 30 November and 1 December strips.
Vlad Dracula the Impaler was a 15th-Century monarch of the
Birgau region of the Carpathian Mountains, in what is now
Romania. In Romanian history he is best known for two things.
One was his skilled handling of the Ottoman Turks, which kept
them from making further inroads into Christian Europe. The
other was the ruthless manner in which he ran his fiefdom.
He dealt with perceived challengers to his rule by impaling
them upright on wooden stakes. Visiting dignitaries who
failed to doff their hats had them nailed to their head.