I somewhat recently came back to UO and had a question about something that was probably changed a while ago. At first dark fathers were vulnerable to both demon and any elemental slayers and they stacked (had an elemental+demon slaying bow and it was fun). Then I know they removed the stacking slayer bonus so you could no longer get the benefit from both. That's about when I had to leave the game and now that I'm back I'm wondering if DF's are still even vulnerable to elemental slayers at all?

They are.

Yeah they still are but demon slayer is much much better than elemental against them.

Demon and daemon slayers work on the Dark Father as "full" slayers (100% more damage, or double).

Elemental slayers partially work (50%).

So an elemental-demon or elemental-daemon slayer would do 150% more damage, and basically became known as a Dark Father slayer.

These first appeared due to a bug with Juka bows.

Reports are that they still work.

But, Dark Fathers Blood Oath you now.

-Galen's player

The blood oath isn't too much of a factor in not using them. It's just hard to find a demon and elemental slayer that has other nice mods on it. Another reason: the slayer bonus is still under the effect of the 300% DI cap.