This topic appeared in the discussion of the HERO’s roster.
You may take one of the Relics from the supplement if your warlord is a cult of strife character. So it is stated in the supplement. The similar thing is indicated in the codex – you may take one relic if your army is led by a Drukhari character.
As HERO said and it is probably true, these rules apply only to the free relic. So if your warlord is a Troupe Master, you may not take the free drukhari relic. But you can do it for CP.
So if your army is led by an Archon, your free relic is the Djinn blade, you may buy the Morvaine’s agoniser for CP.
But there is no limitation in the supplement for characters, who may take certain relics. For example I see nothing, what prevents an Archon from taking the Dark Lotus Toxin.
Please prove me wrong.