Thanks for the replies (@Duality52, @16Bitman & @Slightlyconfuzzled).
I think I need to provide more detail to better explain...
I've already identified all 3 required requiems (Xata, Fass, Jahu). The 1st slot is confirmed as Xata (it is listed as bright white in the attempt list & I am able to attempt a 2nd "stab"). I'm stuck with the 2nd slot. I've tried both Fass & Jahu - both fail. None of the requiems currently installed are Oulls. My last attempt was Xata, Jahu & Fass - which also failed.
I can't insert a screenshot into this message, but hopefully the below link works (link to the screenshot saved on Google Drive). This shows the various attempts to date.
Screenshot
I've had this Lich for a very long time (+1yr) as it's been so annoying, but it's time for it to be vanquished...
Thanks for the reply. Please see my more detailed reply in the post from "Slightlyconfuzzled" which has more details explaining my situation.