I am getting this bug all the time with the Boneyard morph (Unnerving Boneyard). Here is what I experience for others to share their account:
I find the ability is unstable in group settings (dungeon parties of 4 and even participating solo in dragon encounters with other players). This ability will simply not cast when other abilities are able to be used. Once the ability finally is able to be cast, your character will incur what feels like an 80% movement speed reduction. The only way to remove the effect is to die or relog.
This issue is near-identical to a 2014 bug in this forum thread:
https://forums.elderscrollsonline.com/en/discussion/64759/my-character-is-permanently-walking-running-slowly-after-main-story-quest
Perhaps it is this ability or server issues with the launch. Having just ran Spindleclutch I with a group, even non-Elsweyr locations can cause this. (There was a second Necromancer in the party, but they did not use this spell to my knowledge.)
Thanks.