Whenever a town has an associated plot battle, that battle will take place in a node slightly off the town, allowing players to prepare, buy new weapons, grind, and take the battle when they themselves are ready
When Sheoldred enters, each opponent sacrifices a nontoken creature or planeswalker of their choice
For a better idea of what a full list for this Commander and background pairing might look like, go ahead and take a look at Modern Hardened Scales decks
The ability to choose between wiping all dragons or everything else lets you break board stalls wide open while leaving your creatures untouched
You gain life equal to that creature's toughness
Blue: Sea Gate Restoration, Rhystic Study (I mean, it has to be listed here), Pull From Tomorrow, Treasure Cruise, etc