NPC Adventurers in the Megadungeon

Evidence of other, perhaps rival, adventurers in the megadungeon not only gives it a lived-in feel, but also allows for a ready source of rumors to feed the rumor table when the PCs are topside and (somewhat) safe. Encountering said adventurers in the dark corridors provides players the opportunity to do the roleplaying thing, in contrast to the tactical wargame thing.  

Whether or not the DM is actively moving one or more NPC parties around, clearing rooms, finding treasure, or dying in place to leave treasure behind, is game dependent.  Still, fallen adventurers are a great source of unguarded or poorly guarded loot.  Canny use of Speak with Dead can provide PCs with far more than just a coin purse and rusty dagger, while the various Animate Dead spells provide PCs with more sword-fodder.

Even a more mundane analysis of the corpse may provide some clues as to what type of monster may lurk in the area.

Rival adventurers.  With a map! WotC

One of the better forms of loot to find on a fallen adventurer is a map.  It doesn't matter if the map leads to hidden treasure or merely reveals the presence of a secret door or stairs to the next level, a map and the information it provides is worth its weight in gold.

Because in a proper megadungeon, knowledge is power.  Or its at least half the battle.

Granted, some adventuring parties fall prey to TPKs, and in those cases, perhaps one of their hirelings should survive.  This post at Hobgoblinry introduces 'the last lantern-bearer' mechanic, and it is one worth considering for your games. Frankly, all of Hobgoblinry's megadungeon musings are worth reading, so check them out while you're over there.  

PCs hiring these surviving lantern-bearers end up with more gameable information.  

tldr; make use of NPC adventurers, even if they remain off-screen - they are an incredibly useful tool.

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Speaking of rival adventurers and surviving henchmen, my friend Ben (not this Ben) asked me to share his Discord server with the world.  He is also participating in the #Dungeon23 challenge, while serving as a competent online DM-for-hire.  If you're looking for an online game (mostly 5e, but not always 5e), give him a looksee.

Joining his Discord is free, though, as is participating in its various channels.