Forum:Macro cosmos, D. Fissure, Gravekeeper's Servant and Needle Worm

Its simple: while MC or DF are face up on the field and there's a Gkeeper Servant on your side of the field yout opponent cannot declare an attack as they cannot send 1 card from the top of deck to grave.

Now on the other hand when there is a DF or MC on the field and NWorm eff activates the cards are removed instead of going to the grave.

The question is: why for Servant sending it to grave is mandatory while in Worm (and cards like him) isn't?