Headless Skaab

That 3/6 body is the entire pitch, and the graveyard tax is what pays for it. A wall this thick on three mana would be undercosted as a vanilla blocker, so the design fences it behind a self-mill cost: you cannot cast it until you have a creature card to exile, and casting it shrinks whatever recursion or delve plan that graveyard was feeding. It belongs to the self-mill subtheme where filling your own yard is a resource you spend across multiple cards, and Headless Skaab is the spell that competes hardest for that resource: it wants a graveyard, then eats from it. The enters-tapped clause closes the other loophole, denying the wall a turn of immediate defense so it cannot stonewall an aggressive curve the moment it lands. What you get for those concessions is a blocker that laughs off most early creatures and trades up against bigger ones, the kind of toughness-heavy body that turns a race into a grind. The fragility is structural rather than combat-based: nothing on the board threatens it, but every point of toughness rests on a graveyard you have agreed to hollow out.