Tentacle of q'bthulhu

tentacle of q'bthulhu is a rare item dropped from shadow giants and more and can be used for forging

chaos forge
a chaos forge is a forge that swaps the third or forth stat on your equipment. This requires flux and tentacles, but use at your own risk because there is no going back after you chose to randomize stats and you need more tentacles after it is done.

trading
for now one I will just estimate since a lot of time I was probably wrong,

having one tentacle is worth a lot of flux but really hard to get so if you do want to trade it it could lead to a ton of flux, glim, and eyes.