So... that creature stays on the battlefield? Wouldn't that be Regenerate, mostly?

Or is this a super-duper crazy engine card with Ashnod's Altar?

It's no engine card, because this only applies once. It's like somewhere between Withstand Death and a single-target Faith's Reward. And rather expensive for it.

Created for Challenge # 076.

Yeah.. it's "Regenerate target creature"... only; oooh; the shield doesn't wear off at end of turn.

So it's actually most like totem armour. Which is valuable; though could well be downcosted as it doesn't do anything else. (And probably SHOULD be templated as totem armour, as there's nothing on the table to remind you this effect is in play.)

I'd rather play Spider Umbra. The previous ability was better, but should probably only last until EOT.

Indeedy, does seem overcosted in comparison :) (Though I always thought spider umbra was always very good.)

Wouldn't the previous ability be useless against damage and effects that reduce toughness, though?

Good point. The game would try to put it into the graveyard as a SBE, but that action would be replaced by not putting it into the graveyard. Next time SBEs are checked, the game would try again (since it still has damage marked >= its toughness), but this time it wouldn't get replaced. The creature would end up in the graveyard anyway. An alternative would be "The next time target creature would leave the battlefield this turn, return it to the battlefield." That might not work if it would go to a private zone, however.

Probably would have to replace it with flickering.