Aeon's End
Does Phaedraxa's Auspex Rune prevent damage to players, to Gravehold, or both, and can I still pick a damage option?
Both — it prevents all damage to players and to Gravehold for that turn. You may still choose a damage option on a nemesis card; the damage is simply prevented. It cannot stop non-damage effects or loss conditions such as Prince of Gluttons' Devour.