When I run games online the game I most often run is Kalmatta, Island of Adventure. I’ve talked about this on the podcast and even dedicated an episode to it. Recently, I ran the game for a couple new players and a few veterans. The group journeyed deeper into the Obsidian Mounds than any had dare venture before, quickly learing it was inhabited by some form of undead, called Draugr or Odada. They killed the first group of these beasts the cleric having Turned them. What they didn’t realize was one of the turned creatures encountered a more powerful entity, an Odada Wraith. The Wraith led the Odada back towards the interlopers and a final showdown with this group killed the majority of the PCs with only the cleric and a single henchman surviving to return to Sindanore and speak of their companions horrific deaths.
In the B/X system, fatalities are common, but I prefer to decrease the impact of PC death to give characters a bit more longevity. I use a rule from the great little game called Blood & Bronze. I call it Death Boxes. The first time a PC goes to zero hit points, they are down for a few rounds but recover with no negative effects. The second time a character is near death’s door, they will receive a permanent injury. (I use the Mortal Wounds table from ACKS core book pg 106). The third time, the PC is dead, but they do get a single, last ditch action. (So if you’re using this rule make it a good one!)