忌まわしい印章

The copy of the spell is created on the stack, so it's not “cast.” Abilities that trigger when a player casts a spell won't trigger.

You may sacrifice only one creature to pay a spell's casualty cost, and you copy the spell only once.

Casualty N means “As an additional cost to cast this spell, you may sacrifice a creature with power N or greater.” and “When you cast this spell, if a casualty cost was paid for it, copy it. If the spell has any targets, you may choose new targets for the copy.”

If you cast Grisly Sigil targeting a creature or planeswalker that hasn't been dealt noncombat damage this turn, choose to pay its casualty cost, and choose not to change the target, the copy will deal 1 damage and you will gain 1 life. Then, the original spell will resolve. It will deal 3 damage, and you will gain 3 life.

If you pay the casualty cost of a spell, the copy will resolve before the original spell.