While you control The Darkness Crystal, abilities that trigger whenever a nontoken creature an opponent control dies won't trigger because that card is exiled instead
However, its usefulness may be limited in decks that do not heavily rely on Synchro Summoning or Assault Mode Activate cards, as it may not provide as much value in those contexts
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Its also worth noting that unlike Creature Swap (a similar card that allows you to swap any of your Monsters for an opponents), Creature Seizure doesnt prevent you from changing the battle position of the Monster your opponent gives you
This could be used as a tech against the opponent by running the appropriate Shaddoll monster Shaddoll Dragon to destroy 1 Spell/Trap card, or Naelshaddoll Ariel to banish up to 3 cards from the graveyard
To those soldiers braving the battlefield that has become Pokmon card collecting, I wish you good luck