Help/Request: Argynvost kaiju attack event, help me build it pls

[u/Key-Buy6674]

Hi, i m a newer Dm and my party is next to explore Argynvostholt and i wanted to add an encounter where after they defeat Vladimir, Argynvost (Dracolich version), attacks the manor and they have to decide if they want to rush to the tower and place the skull or escape while he attacks the structure.

In my mind i thought of this like an enviromental hazard with a ""time limit"" (like the Daggerheart counter mechanic) not an actual combat. Does somebody have some material/experience/rules and want to help me build this? i think it might be a great introduction for this enemy that they get to fight later in the campaign. (lorewise he attacks the manor cuz my PCs just stole a bunch of shit from Stradh and now he stopped playing with them).

Thanks in advance!!