Sacrificial Pyre is a very powerful Epic Event, but it requires a lot of setup to really make full use of it.  However, you don’t actually need a high payoff to play it. If you manage to get just 2 boosts, it pays for itself and gives some return on the investment. Two boosts means a new Undead Warrior, which is worth about 3 magic.  Sacrificial Pyre only costs 2 magic total (1 for the card itself +1 for the actual cost on it).  Getting 1 Pyre per game to 5+ boosts is not unrealistic.

You probably should play both Sacrificial Pyres every game as Ret-Talus really does need it but the timing of when to play them is the tricky part.

It’s important to remember it also triggers on the opponent's turn if they kill your units.

A big part of Ret-Talus' strength is pulling off a very big Sacrificial Pyre turn.