It also can be upgraded to tether ender creatures, to have increased damage, to poison creatures, and has a very expensive player upgrade (the previously mentioned soul catalyst).
However, the wiki/readme only mentions one upgrade (other than loading an unfinished blood gem to automatically fill it), that being quartz for increased damage. Poison, tethering, and the 'player upgrade' are not mentioned at all. I'm pretty sure that spider eyes are the poison upgrade, but I don't know what the item you use for ender tethering (ender pearl/eye/bone?) is, or what the 'player upgrade' does (causes mobs to drop xp and player drops, I guess). This information should probably be part of the documentation of the mod.
On the Soulus curseforge page, you say: