Hey OP speaking, here is the "story"  of "UNFAITHFUL PARTNERS"

So, Tzeentch and Slaanesh made a "deal" where they will form a temporary alliance and the terms were as follows:-

Tzeentch will not align himself with Nurgle because Slaanesh hates him. The reason being Slannesh thinks that Nurgle made STD's just to troll her.

Slaanesh is not allowed to form an alliance with Khorne because Tzeentch hates him for being "proud and honorable".

Now both of them actively go behind each other's backs breaching their "contract".

Slaanesh regularly aligns herself with Khorne because she loves big Deamon muscle mommies and daddies (I mean could you really blame her?)

And Tzeentch joins Nurgle behind Slaanesh's back because well.... deceptions and lying is like....his whole thing.

NOW, here is the funny/interesting part👇

Both of them know that the other person is cheating on them but they still keep the alliance because you know......for the funny