Hmm...
Spoiler
I don't have any artwork to make it into a ring or necklace. Using it as some kind of ioun stone (it's not really a helmet) is probably the most appropriate choice for that item anyway.
Immunity to haste is correct, mostly because I've always imagined undeads as generally slow moving. But I could include immunity to any kind of movement speed change in general. I'll think about it. The ability to equip it is more of an easter egg. No sane person should actually do it, unless you are suicidal.
it was my powerplay part talking.