Converting an altar is made easier to do in terms of RNG (either increasing the odds or making the conversion guaranteed after some number of sacrifices), but in compensation for that, all attempted sacrifices cause the altar’s god to get angry at you and smite you, with the penalty dependent on your XL or level difficulty. They can do things like curse your items, give you negative protection, send minions to defend the altar, and blast you with lightning and disintegration if at a high enough level.