Wowsers. I guess for Death Mutation mana, you deserve to get something splashy. I'm somewhat scared of the Delve option meaning this could be cast rather more cheaply though.

As there isn't significant data to judge what's reasonable for costing Delve cards, I'm definitely just winging it a bit.

For all common delve cards (Revoke Visa, Hollow Bones, Six Low Notes, Truffle Hog, and Pinning Gale) you're getting something that's slightly above the curve if you max out on Delve, but you have enough room that you can fit the cost to the status of your graveyard without feeling like the spell is overcosted. i.e. It only takes 2-delve to change Revoke Visa into Regress.

I think the big issue is where I've tacked on dramatic ETB effects to delve creatures, effects that are significantly above the curve if you max out on delve. Put this at like... might be the reasonable thing to do... but that's also really goofy aesthetically.

For better backwards compatibility and general refinement purposes, I'm going to experiment with changing all the Mozgul into Saprolings. I never realized that Saprolings had been isolated to a token-based existence.