Hi, when card “Grave Digging” is buffed with deathrattle effect of “Hunting Nighmare” and is played next turn with reduced cost because obviously “Hunting Nightmare” is undead type which reduced cost of “Grave Digging” then deathrattle effect from “Hunting Nighmare” is not summoning the 4/3 Soldier.
Edite:
“Hunting Nightmare” deathrattle effect not working also with “Undying Allies”.