Weak Blood Shard

These shards become really important later on in the mod (as of Altar T4) but they can also be used to craft a few potions and decorative blocks such as Blood Stone Tiles. The only way to get hold of these is to kill a hostile mob with the Bound Sword (while it's active).

This is only a small chance but you will need a fair few to progress further into the mod so get farming and make sure that you keep your LP network topped up.