Ring of Xathrid is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: Regenerate equipped creature. (The next time that creature would be destroyed this turn, instead tap it, remove it from combat, and heal all damage on it.) At the beginning of your upkeep, put a +1/+1 counter on equipped creature if it's black. Equip (: Attach to target creature you control. Equip only as a sorcery.)