Go to misc (miscellaneous) and it will kinda look like a shoe. That is the item that you use to extract blood. When you have a decent amount of blood packs, you can find a cave on the lava island by the coordinates of 90.7 lat, 78.6 long, facing E (east). This is the cave I normally go to to tame desmodus. When you walk in, there will be lots of onycs and bugs, so be ready and bring some antidote and bug repellent. THE DESMODUS HANG FROM THE ROOF you will not immediately see them (notice I used pluralism). The desmodus will latch onto you and carry you into the air. It will then eat (drink?) the blood packs. It will then be tamed. I hope this helped! 👍👆
Ok, so to get blood packs you need to make a blood extraction syringe and use it while its in your inventory. This will damage you though, as it is taking blood from you, so dont make too many at a time.
Post more useful ways to gather blood packs. If we need to learn how to tame it we can go to the Desmodus page. Up so we can stop filling clutter posts on how to tame it. The flying in bug caves and only attacking bugs was the only useful comment here