A Lion in The Garden -Tywin Lannister-

There are very few things-or nothing at all, if you were to ask her-that (Y/N) Tyrell hates more than Tywin Lannister. And yet, drastic times call for drastic measures, and the War of the Five Kings has displaced nearly every single house. When Lady Tyrell finds herself entering into an alliance with House Lannister for the good of her own family, and to defeat the common enemy of Stannis Baratheon, she is certainly reminded of just how insufferable she finds the Lord of Casterly Rock to be. However, a rather dangerous situation during the Battle of Blackwater, and her forced proximity to the Old Lion, makes Lady Tyrell begin to consider the thought that perhaps Tywin Lannister was not so horrible as she previously thought.
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Tropes (at least the ones coming to mind): Enemies to lovers (also sort of friends to lovers), Forced proximity, Secret relationship, Age gap

Warnings: I mean it's a GOT fanfic, so expect gore and sex. There are also mentions of SA and rape, so please be aware of that.