The Terraria 1.4.5 update will let you transform into a vampire bat, and gives you the opportunity to place crimson and demon altars.

Terraria, a game based around crafting, explorations, ... Opening the Crafting menu next to a Demon Altar allows you to shape six Lenses into a single Suspicious Looking Eye.

Terraria at IGN: walkthroughs, items, maps, video tips, and strategies. Focus Reset ... The more straightforward path is to seek out a Demon Altar in the course of your explorations.