Pwnhammer

The Wall of Flesh always drops it, and it always does the same job: smashing demon and crimson altars. Each altar you break seeds a batch of new ore across the world, so this hammer is what stands between you and every hardmode tier of gear. Break a few, go mining, come back and break a few more when you need the next metal.

Altars only break in a world that has already gone hardmode. Carrying the hammer into a fresh world and swinging at an altar there accomplishes nothing, which catches people out when they try to shortcut a new save with old gear.

It comes from a Metroid themed level pack Redigit worked on before Terraria. Pwn is old multiplayer slang, born from a typo of own that stuck because the two keys sit next to each other. The color scheme matches the Hallowed tier, incidentally, despite the hammer having no recipe of its own.