This may come across as a silly question. But how many Coven supplement artefacts, the Diabolical Playthings, can a single Coven HQ use?
It says "Any units... that can select Artefacts of Cruelty cannot select from those listed in Codex: Dark Eldar, but can instead select Diabolical Playthings, presented opposite, at the points costs shown."
And further on under Diabolical Playthings it specifies "Only one of each of the following relics can be chose per army..." but does NOT specify that a given model can only select "one of the following".
In Codex: Dark Eldar, it is the Artefacts of Cruelty list in the war gear lists section on p.69 that specifies "A model may take one of the following:"
Now my interpretation is that since you aren't selecting from the Artefacts of Cruelty list, but from the Diabolical Playthings list, that this restriction is no longer included.
I dedicate these deaths to Odin Allfather, Spearshaker, One Eye.
Fleet Shadowmaker - Kabal of the Dying Sun; Cult of Marrow Excised; Coven of Lambent Hunger
Sons of the Last Breath - Chaos Space Marines
Host of Shattered Purity - Chaos Daemons