The Black Star

If you control Azura's Star, sacrifice it as an additional cost to cast The Black Star.

Soul Trap — Whenever a Legendary
Creature or Planeswalker dies, you may tap
this artifact. If you do, you may search your Graveyard for target Enchantment and cast it without paying its mana cost. You may not activate Soul Trap with another Soul Gem for
this Creature or Planeswalker on this turn.