I think making it post EoW/BoC is the earliest something like this should be. It shouldn't be available at the start, but by Hardmode, you're not using Altars to craft boss summon items outside of resource/money farming. Placing it after the evil bosses makes it so by the time you can get it, you'd be very capable of surviving the biome anyway, and have in fact "conquered" it. The main reason I see that this should be implemented is to make it so you don't have to venture out of your way/build housing in extremely specific spots to take advantage of an Altar spawn outside the evil biome.