Vampire Lord Abner is a short 4-page pdf (not counting the cover and legal text) that details a single encounter: what is likely to be the final encounter of an adventure. In it, Jay Holden dispenses with all the preparatory encounters and just goes for the main event: a fight against a vampire lord, leaving it to the DM to fill in the rest.
This is actually more useful than it may appear: designing good final encounters is hard. They need to be memorable, and last longer than a single round. Of course, the existence of spells like Otto’s Irresistible Dance do make them more difficult to pull off, especially against single foes. If you’re going to have a battle against a single opponent at higher levels, make sure it’s immune to charm spells or has Legendary Resistance. Abner has Legendary Resistance, but not the immunity to charm – something I’d advise adding.
Jay has taken an interesting approach to the encounter: Abner changes his statistics and tactics as the fight goes on. Three stat-blocks are given, each of which has a third of the hit points you’d expect the Vampire Lord to have. I expect the intention is that Abner moves from one to the next when the players reduce the current stage to 0 hit points; this instruction was unfortunately left out of the text. This is an effective method of showing how Abner changes tactics as the fight proceeds. 4E experimented with some abilities only becoming available when a creature became “bloodied” (reduced to half hit points), this extends the process in an effective manner.
The Vampire Lord isn’t just a set of statistics, however: he has a history revealed through his actions and possessions, some of which the players might discover if they pay attention to his actions and what he leaves behind. The hints work better than it being fully stated, in my opinion.
Mechanics-wise, there are a few strange points, although none of them are actually wrong; they just work differently to how other monsters are presented. More significant is Abner’s use of bonus actions: Abner gets the choice of using a powerful Superiority attack or making an additional attack with an off-hand shortsword. Why the limitation? This isn’t a player character: have him do both. It’s not like this phase is going to last very long, especially if a cleric and wizard are involved in the fight!
The presentation of the monster has a couple of quirks: the paper size isn’t consistent throughout the document, and not all the abilities are written out in full on each page. It’s a space issue, but I’m not convinced the formatting is quite correct.
Overall, there are some very good ideas here, although there are a few niggling issues. Even if you don’t use the monster as written, some of the mechanics used could inspire more encounters along these lines.