Steps to Tame a high level Desmodus with 100% Effektiveness (you need a low level Desmodus to start) :

Step 1: Tame a Stego/Andrewsarchus and saddle them (the saddle on the Andrew is essential!)

Step 2: Tame a Moschops of any level

Step 3: Farm 400-600 Bloodpacks

Step 4: After finding your target Desmodus take out your Stego/Andrewsarchus and your Moschops

Step 5: Put all the Bloodpacks except for the last 100 in the Moschops (the other 100 you keep in your own inventory)

Step 6: put the Moschops on "Following: Lowest" and let it follow your Andrew/Stego

Step 7: Aggro the Desmodus by approaching it and let it grab the moschops, the Desmodus will eat 10 Bloodpacks from the Moschops' inventory and then drop it, this will raise its Effectiveness by 1-3% (repeat this until the Desmodus is at 100% Effectivness. Do not unmount while doing this!)

Step 8: once the Desmodus is at 100% Effectiveness unmount and offer yourself up to the Desmodus. It will grab you and eat 10 Bloodpacks and then drop you.

Step 9: after it dropped you immediately mount your Andrew/Stego again. And let it grab the Moschops again so it's Effectiveness goes back up to 100%

Step 10: Repeat Step 8 & 9 until the Desmodus is tamed.

This will allow you to get perfect tames.

In Fjordur the spawn locations are :

Latitude:38 Longitude:75, inside a cave under a waterfall

Latitude: 90 Longitude: 78, cave as well

Latitude: 50 Longitude: 14, cave

Use a Daedon, put meat inside, set to passive, passive healing on and kill all the other desmodus until there is only one, then duck under the Daedon and the desmodus will grab it, suck it's blood and the effectiveness will go up 3-4%, don't leave the area, hide and wait until 100%. Once 100% effectiveness, let it grab you (make sure you have 150-200 blood packs) and you'll tame it. Use cryopods if possible to bring back the Daedon.

If you want a high level, get a moschops, level it all into health, find a 140-150 desmo, lure it somewhere, get it to grab you with some blood packs, and a sanguine elixir, as soon as he’s about to grab you spam the elixir that’s on your hotbar and start the tame. Throw out your chops once he drops you, stuff some blood packs in and spend the next hour letting the desmo grab the chops over and over till 100% and then let the desmo grab you to finish taming. - may have to get it to grab the chops again to get back up to 100%

For a decent level one you only need some 40-60 blood packs, and just have them in your inventory, not in your hotbar(although you can have them on hotbar) and then agro it. I use a longneck, but you can just use a bow. Once it agros let it get you. It will grab you and carry you around, draining blood packs, and taming. The game will tell you to attack to escape but don’t do that it won’t help the taming. If your looking for some on fjordur, then from the mainland go to the snowy mountains across the ocean and look for hexagon pillars of rock, forming a canyon, exactly like a wyvern trench. There will be 2 cave entrances, take the one on the right. You can do this at any level as long as you have:

A flyer to get there and back

Blood packs.

If on x1 taming rates then you should only need ~50 blood packs for a level 100.

Try to tame a low level one first so you can farm blood packs with it, it'll shorten the farming time for a high level bat.

You can also tame a Daeodon first and use it's healing aura when extracting the first few packs, the pig can also help you raise the taming effectiveness for future high level tames, just let it wander on passive near the Desmodus you want to tame, keep some blood packs on it's inventory and let it get picked, it'll raise the effectiveness.

Let it pick you afterwards and feed on your blood packs to raise the taming meter, when it's about to tame, be sure to check if the effectiveness is close to 100%, if not just let it pick the Daeodon some more.

To tame I use a snow owl to heal while harvesting blood then I tame two mosschops put lots of blood in there invintories put them in passive let a low level bat it argo on them and pick them up then it should have 100% taming affectivness harvest blood with that rinse and repeat. Good luck Randle out.