The "on drop" trigger in the map editor has a field to select Underlord. The tooltip for it just says "The underlord in question", which is nonsense, because there are multiple underlords in question.
So I tested it, and it turns out that the underlord doing the dropping and the owner of the unit must both be the underlord selected for the trigger to fire.
At the very least, make the tooltip reflect that both the dropping and owning underlord must be the selected underlord for it to work. Even better would be to make it able to select one or both underlords independently.
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