[mayegelt]

Yeah

When choosing your army, you must nominate one model to be your Warlord. Unless specified otherwise, this must be a character model. If you do not have any character models in your army, then select any other model in your army to be the Warlord. The model you choose as your Warlord also determines your Primary Detachment.

This also meant that I actually had in a recent game the leader of my army being a Helbrute Murderpack Champion. He was made leader because the army was a Murderpack, 3 Mayhem Packs and 2 Maulerfiends (+ an Aegis with comms and a Voidshield Gen). Though I would have preferred one of the Mayhems to be leader, but the Murderpack had to take it as a Char. Same would have gone for Cultist Champ if I would have gone for a Helcult (and what an easy char for slay the warlord to get).

Oh in addition to this one of the fun things is that a Wraithlord can be your leader in an Eldar army as he is a char. Stupidly the Iyanden book tells you as a special rule they can be your warlord even though it was never required.

 

[mayegelt]

to be fair, Iyanden was released in 6e. And was the first supplement, actually, when they didn't exactly have the wording right, so didn't even allow allying Iyanden to Codex:Eldar (like later supplements did to parent codexes--for all that detachments in factions have shifted around, now, in 7e).

Well TBH there was no point in allying Iyanden to Eldar as Iyanden was the same as Eldar with an extra set of optional Relics and a couple of rules that gave them more options (aka more spirit seers if you want) but with no draw backs.

 

[mayegelt]

But in the case of the CSM / CS one, each can ONLY take from their own pool of relics. The Iyanden one allows you to take from either in an Iyanden army. Just not from both on the same character. So I could have an Iyanden Spirit Council with 2 Spirit Seers 1 has Wraithforged Stone, the other (still from the same HQ choice) has Mantle of the Laughing God.
Literally the only drawback from Iyanden is that the Spirit Seers HAVE TO take Voice of Twilight, rather than Conceal/Reveal as primus.
Otherwise they get access to the Iyanden Warlord traits (if they want) as well as the Eldar ones.
Spirit Councils mean they can take 1-5 seers as a single HQ choice (rather than just 1).
They get access to both Iyanden Relics and Eldar Relics (just not on the same char).
They have the additional option of having a Wraithknight as leader.
They get additional Cities of Death / Planetstrike Stratagems.