1. Will felar be able to wield something very heavy like Hammer of Muspellsheim (weight - 35)? Will felar still also hit with claws if he can wield such heavy weapons?

2. How to calculate the maximum weight of heavy one-handed weapon (like mace) felar can wield? What about two-handed weapons (like spears)? Is there any difference in weapons for felars?

Might have been Fordurn, but there was some 2h mace that my felar thug used.

It shows all the maximum weights.
Fist of the titan, if you have it, adds another +10 pounds on top of these. So the absolute limit for a felar warrior is 32. And yes, the claws will always be there unless you wield a one-handed weapon and a shield.

Also, keep in mind you will be quite easy to disarm via strength loss.

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