Make it so that the PC cannot be sacrificed on the *very first turn* when he steps on an altar on D:48. If he lingers or steps on it again, only then should it be dangerous.
Altars aren't that rare, really. Is it like a 1/10 chance for one to be generated? Maybe even more common? It's really easy for a newbie who's exploring the level for the first time to assume a light gray underscore represents an air rift, or a dark gray underscore is an earth rift or mana or whatever. I just think there should be a safety mechanism here, or just make the rifts more distinctive. Change the ASCII character, or turn the surrounding tiles into colored ones, so that a water rift has 8 blue "." tiles around it etc.