When you don't have an instant or activated ability, Relic of Legends second ability does not hold priority. So, when my legendary creature gets targeted by a fateful absence, I am unable to tap it for mana (that I would then use to activate the clue) before it dies. I don't know if there are other corner cases like this, but I would suggest giving priority to the Relic of Legends player when one of their untapped legendary creatures is targeted.