Oops...forgot...we'll retcon that attack from the hook horror (I editted it away). Do you want everyone to back away before they go. One of them will get an OA on Orsik or Derendil. You can move back 12'. if you do the one that didn't fall can move and attack. the one that is 20' down can only dash to get back within reach.
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They can't disengage; they both attacked. (They can only move 12' because the terrain is rough.) This is why I said I thought Orsik should disengage instead of hitting HH1, but oh well.
I still think they ought to take the OA and run. Reasoning: If they run now, they'll take one OA, then one hit when the HHs go (since, if they move, the fallen HH will have to dash to catch them).
If they stay, they'll take no OAs, but then two hits when the HHs go (fallen HH can still get to their current spot without dashing).
Either way, two hits, but if they move now they'll be 12' closer to favorable ground.