Halana and Alena doesn't cause a prompt before combat like Luminarch Aspirant
Unless I'm somehow sorely mistaken and confused, when your opponent controls a (non-alchemy) Luminarch Aspirant, the game stops to ask for input at the end of precombat main phase, to allow you to act before its start-of-combat trigger. (I remember this happening to me without manually placing stops, when I'm not force-passing to end-of-turn, as long as I have an instant or activated ability to use)
In an alchemy game I just played against someone playing Halana and Alena, Partners, the game did not prompt me for input despite holding Doom Blade and Putrefy (from 2 Key to the Archives), I wasn't able to react until the trigger was on the stack, which obviously is too late to prevent the trigger and its effect. (Thus losing me the game, the reason I noticed)
This is the first time I've noticed a difference like this, so I don't know if it's something specific to H&A, the version I'm on (2021.12.10.3971.904583), my cards in hand, or what. Reckless Stormseeker was in the same game and also has a beginning of combat ability, but the stakes were never so high that I noticed a stop or lack thereof for its trigger or any previous H&A triggers.