Extra mana for no apparent reason

A recent blog post clarifies this, the exclusive DLC consists of four familiars, three of which have slightly elevated stats for their level, and the fourth is a monkey who shoots enemies with his banana-guns... Yeah.

Here's from the wiki:

• It is possible to bank mana during the building phase using the forge. (Only after first wave.)
• Items left on the ground at the start of a wave get turned into mana and split between the players.
• End of Level - At the successful completion of a level, you will receive some mana back for your towers.
• Upon leaving a level - Items on the ground are sold and split between the players
• Extra items can be sold for mana based on their quality.
• Mana can be traded between heroes in the tavern by dropping it.
  • Access the Hero menu and choose "Drop Mana".
  • Mana has a maximum of 100,000 per Drop.