When you equip Volendrung, it appears to be resetting your armor and also your Weapon / Spell damage. With 5 Spriggan, 5 Automaton, and 2 Troll king, with 3 infused jewelry with 2 stamina recovery enchants, and 1 Weapon damage enchant, I was at 3666 weapon damage before picking up the Volendrung, and after I picked it up I was at 1198 with Major Brutality buff. Here is a screen shot of the Weapon damage. Also the Maximum magicka is scaling very very high on a stamina based character, I went from 10k, to 46k. Meanwhile my main stat which is stamina went from 37k to 40k. This seems to be scaling the off-stat higher then your characters main stat which is very odd.
In this picture you will notice my weapon damage, and spell damage. Spell damage is 0, and my weapon damage is 960. Another interesting thing about this is my Equipment bonus is 2 / 5 stars even though all of my gear aside from my head / shoulders is Legendary, aside from those 2 pieces I just mentioned. Hopefully this gives some information needed to fix the Volendrung.