What happens when you cast Wormfang Manta and it dies before your next turn?
Basically nothing.
When Wormfang Manta enters the battlefield, you skip your next turn. Effects like this that want you to skip a turn (or part of a turn) wait for the next time you would begin a turn (or a part of a turn) and replace that turn with nothing. The skipped turn will be whichever turn you would take next, not just your next normal turn in the turn order.
When Wormfang Manta dies, you take an extra turn after the current turn. Once the current turn ends, you’ll begin that extra turn.
If this Wormfang Manta’s second triggered ability resolves before you’ve skipped a turn for its first triggered ability, then the turn that will be skipped is that same extra turn that’s created by the second trigger since it’s the next turn you would actually take.
