What the heck is this?

[Sailis]

So I've been playing terraria for a bit now, and love it I found something in my wanderings and i have no idea what the heck it is

Any info would be appreciated

 

[A (Pioneer) Rock]

Pretty sure it's a blood altar from thorium but I could be wrong.

 

[A (Pioneer) Rock]

it allows you to summon a boss with 5 unholy shards

 

[Sailis]

Awesome! Thank you man, I really appreciate it