Does sacrificing a unit in Malekith's forced action enable a DE player to place power on any and all Slave Pits if in play ? What about Whip the Slaves tactic ? Would that also allow power added to Slave Pits if the tactic is played ?

I think I have an answer to my question . Thinking over the context of the cards involved with Slave Pens and the tactic Whip the Slaves , they all involve a single Action utiilized during an action window . I beleive the action on Slave Pens and Whip the Slaves need their own individual sacrifice to trigger the reward . So two Slave Pens in play and a Whip the Slaves would all need 3 individual unit sacrifices to get the desired effects .

Malekith also has an Action where each player sacrifices a unit and he can benefit many DE card effects when an opponent discards cards and in particular units . I'm not sure I find him the best DE Legend to play but thats just my personal preferences .

any thoughts ?

The resources had to be put on them, if textbox said: when the unit is sacrificed. So you're correct, to put tokens you have to sacrifice a unit for every Pen.