"Response: After you engage an enemy, add 1 resource to Mablung's resource pool. (Limit once per phase.)"
After you engage an enemy includes also the action when an enemy engages you (because of threat or other card effect) or it refers ONLY when you make an optional engagement?

Both. There is usually no distinction between optional engagements, engagement due to engagement checks or engagement due to card effects. All of these would trigger Mablung's effect. This is true whether the game refers to you engaging an enemy or an enemy engaging you.

If there is a meaningful distinction, it will be explicit, such as the enemy "Zealous Traitor" from Heirs of Numenor: " Forced: When Zealous Traitor engages a player, that player must deal 1 damage to each ally he controls. (2 damage instead if Zealous Traitor was not optionally engaged.)"

1 hour ago, bitzurock said:

"Response: After you engage an enemy, add 1 resource to Mablung's resource pool. (Limit once per phase.)"
After you engage an enemy includes also the action when an enemy engages you (because of threat or other card effect) or it refers ONLY when you make an optional engagement?

Every time you engage or are engaged by an enemy with the limit of once per phase. Meaning that with proper shenanigans you can trigger his ability multiple times in one single turn, like planning, questing, encounter, combat and refresh phase... all are very unlikely unless you devoted yourself and your deck to this single purpose but 2 or even 3 are quite easy to achieve in a deck using the right cards and especially in combo with like tactic aragorn.

Mablung is your Las Vegas hero card equivalent....

Ok. Thank you!

Just to clarify, this only applies to enemies that “physically“ get into your engagement area and not for enemies you are “considered to be engaded with“.