With her players now down on level two of her home brew megadungeon, the chances for fatalities has gone way up, but natural segues for introducing fresh characters aren't as easy to come by.
Here are a few ideas we tossed back and forth:
The fresh character ...
1. ... is held prisoner by the goblins, lizardmen, or whatever has set up shop in the dungeon. Freed, she joins her rescuers.
2. ... is discovered as a tiny figure inside a corked bottle, trapped until bottle is broken.
3. ... was turned to stone until a secret command word is spoken aloud (which the party just happens to say while investigating the room).
4. ... is part of a mosaic or fresco. She springs to life, along with several monsters, when the crystal orb in the room is touched.
5. ... is found stuck in a trap (a covered pit, magnetic wall, etc) until she is freed by the party.
6. ... is a goblin, young lizardman, or a sentient living skeleton who befriends the adventurers.
7. ... is an intelligent, talking giant spider (treat skin as leather armor) who has recently moved into the dungeon.
8. ... is an adolescent minotaur or other big bad ... starting at only 1 HD, each time it levels up it grows stronger and more dangerous.
9. ... is a member of another crew of adventurers after the same goal or just in the dungeon looking for loot.
10. ... is a deserter or other wanted man.
11. ... is a minor retainer, link boy, or groom who has been watching the horses and supplies up until deciding to brave the depths to find his master and deliver a message.
12. ... is a characters who came along on the expedition from the start, but who was struck by a fever and has been laying about camp on the surface recovering. Only now has she descended to join her fellows below.
As lethal as Basic D&D is, it seems like suggestions for getting new PCs up and running in medias res would come as part of the guidance offered to new DMs.