When to sacrifice heroes, which heroes you can never sacrifice, the reincarnation groups and reclaim recipes, and how many shard packs a hero is worth.
The Altar takes heroes you no longer need and gives something back. It opens at map floor 400. A second, stronger Altar for shard packs opens at floor 1500.
These heroes cannot go on the Altar at all, so gear and rarity you put into them stay for good.
Each group below is a set of heroes that can turn into one another at the Altar. Gold frame = the form most players keep.
Some heroes can be brought back after you used them for a reincarnation. Sacrifice the two heroes on the left together; after the Dark Ritual you get the hero on the right and both of the heroes you put in.