D6 Ways to Introduce Your Next NPC, or Party Member After a PC Dies in the Dungeon

1. They are found trapped in a barrel for monsters to cook/eat raw later, Laurentius-of-the-Great-Swamp-style.

2. They emerge from a scroll, covered in paper cuts. All they can recall of the experience of being trapped inside Scroll-space is something simply referred to as "the Horror".

3. One of the monsters you are fighting is actually a dude/dudette in disguise that learned the language and lived among the monsters for 1d4+2 months (or years). The disguise is incredibly apparent to the party but not the monsters. They are happy to be back with (relatively) regular people but feels guilt occasionally for how the party murdered their "family".

4. The great cosmic Ur-Dungeon, searching for horrors and treasures in the multiverse to populate its deadly labyrinths, accidentally swallowed and regurgitated a being without putting it through the necessary mental conditioning for a dungeon environment. They are scared and very confused.

5. They were "always there" in the group as far as anyone can remember, Rick and Morty style. Everyone acts like this is normal (except for maybe at least one PC who realizes this anomaly).

6. A big monster (like an ogre or something) is walking its pets on a leash. Roll randomly for 1d3 "domesticated" monsters and then add the NPC. Their equipment is probably in the bag slung over the monster's shoulder.

The Ur-Dungeon Denizen Acclimatization Programme: 
He'll be hoarding treasure and killing adventurers in no time!