Pretty important to note that in order to receive the extra levels for efficiency you need to "feed" it a passive small creature before/ during the taming process. Each time it feeds on you it loses efficiency I don't know what tame is best used but I am using a Moschops and it is working well.

This thing literally picks me up when I was away from my screen. Luckily I had enough heath and it was just taming while it was caring me around and stuff so I suggest just standing where it’s at and just let you carry it away but if your health is not over 1000 then don’t do it.

My friend: you tamed that low level Des?

Me: I did, still not craft the saddle for it so it's in cryopod

My friend: HOW? The blood bags are still in the fridge.

I smiled as she shocked by the massive red messages in tribe log.

You don't need blood packs to tame a Desmodus, you are the blood pack yourself. Just pack 3 beds and sacrifice yourself to the low level Desmodus so you can have blood packs for higher levels.... or just do it the normal, painless way.

Get a good pteranodon, build a trap made of double doorways and a gate in the front, use the pteranodon to lure the bats into the trap(Pteranodons can fit through double doorways but bats can’t) net gun all the bats, close the gate, kill the low level bats, then tame the one you want.

1) get good armor so it’s attacks don’t kill you

2) get a bunch of blood packs, dying and respawning with full health is faster than healing it all up

3) actually find one, recommended is a spyglass o and a good weapon or a strong tame to take out the bugs in caves, I usually go to the emerald trench and look for them on the ceiling in the wide-open cave

4) Find a good level and kill its friends. Shooting it with anything will get its attention.

5) DONT ATTACT THE ONE YOU WANT TO TAME! instead, let it grab you and eat your blood packs, this raises the taming bar

6) be careful not to take fall damage, and repeat the grabbing and bam you got a Desmodus!

Be careful out there!

For modded official Tharat:

For anyone else seeking answers in future. Its normal desmodus taming method. Either keep blood packs in inventory and let it pick you and eat those.

Or put a bunch of sleeping and keep feeding yourself to it.

Tip: dont summon it in open with wild dinos. You wont get much attention and it will keep losing effectiveness.