If this would die, exhaust it and remove all damage and attachments from it instead. You can't sacrifice it.

Indestructible Rulings

If an indestructible thing has 0 HP because of having -1/-1 runes, it will remain exhausted forever. If it gets any damage or attachments in this state, immediately remove the damage and/or attachments.Sirlin, 03/19/16
Attachments are cards that attach to a unit or hero, like "Spirit of the Panda" or "Entangling Vines". Runes and other ongoing spells or effects are not attachments.Sirlin, 03/19/16
Even if it has -1/-1 runes that were from something that "dealt combat damage in the form of -1/-1 runes," you still don't remove them when the indestructible thing would die.Sirlin, 03/19/16
Some effects like Obliterate, Sacrifice the Weak, and Death Rites ask a player to destroy or sacrifice the unit that is the least according to some ordering. These effects skip units with Indestructible and units that cannot leave play.sharpo, 11/24/16