@Baudin Toolan, do you know (or could you find out) why Attack of the Hydra appears to have permanently replaced Hades' Portal?
Attack of the Hydra was initially explained as a reskin or slight rework of the Hades' Portal event. For the majority of other events where multiple "skins" exist (recipe events, wheel events, Slinger/Assassins events, etc.) we still see both versions in use.
From what I remember, there was a significant quantity of negative feedback around the changes made from Hades' Portal to Attack of the Hydra:
The event completion reward for Attack of the Hydra is half what it was for Hades' Portal: 3x +100 Land Expansions instead of 6x. (Which turned out to be part of a heavily-disliked move to time the event with purchase packages full of Land Expansions.)
The event is overall much more difficult to complete and actually achieve that end reward:
Missions take significantly longer.
Missions have a significantly reduced chance of successfully finding the objective.
Missions have a significantly increased chance of your troops dying.
Missions require more troops.
The increased event duration does not compensate for these difficulty increases.
The favour and resource rewards appear to approximately scale with the mission time increase, which on its own seems alright, however this doesn't take into account the increased troop costs and likelihood of dying.
Given that this format is one of only two zero-gold events and therefore difficulty increases won't increase gold usage or profits outside of any package releases (which only target a very small portion of the community), I can't quite see the reason for the permanent replacement.