Hraesvelgr's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack.
2025-06-06
Once a creature has been blocked, Hraesvelgr's last ability won't cause it to become unblocked.
Flying, vigilance, ward
Shiva's Aid — When Hraesvelgr enters and whenever you cast a noncreature spell, target creature gets +1/+0 until end of turn and can't be blocked this turn.