Enga's purpose in Mokmurian's forces might be tough for the players to discover normally, but if they encounter her during one of her regular trips to demand tribute from the local redcap tribe they'll have some idea of the part she plays.
In introducing this encounter, describe a short conversation between the kobold and the fey - her demands for gold, their reluctant, begrudging compliance, and so on. Unless the party is taking care to hide (with Stealth checks), Enga notices them nearby and orders the redcaps to attack (as though they needed the encouragement). I suggest the following encounter for a party of five adventurers:
If the party has fallen behind in terms of treasure over the course of the adventure (by failing to recover stolen Scarnetti goods, for instance), this is a good place to give them a boost. Simply include some of the missing gold as part of the redcaps' tribute to Mokmurian.