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.