Really glad you're having fun with the mod! Huginn and Muninn are meant to be for only a single turn. At the start of combat you pick either one and get the effect for that turn only. It basically boils down to gain x effect on the first turn of combat, or when a card says you can gain the effect for a turn.
So the upgraded Huginn card gives you the perk for that turn and doubles it.

These are extremely fun characters but can you explain how Valkyrie's ravens work? I don't understand how Huginn and Muginn work at all

Hi, I am playing with these characters right now and had a question about Valkyrie. When it says to gain Huginn/Muninn is that supposed to be considered a token that you spend? For example, spend Muninn and give all other players an energy. The reason for the confusion is Huginn's upgraded card stating "Gain the perk and double its effects" making me believe that its just a passive benefit.

Otherwise, I am having a lot of fun exploring the game! Thank you for the content.