Was looking for a similar ability to 
Realm of Dwarrowdelf, for a copy-paste-adapt potential solution. Instead, found a bug in 
Uruk Slayer's code: limit is wrongly applied to cost (remove twilight) instead of effect (increase strength). The difference is important if 
The White Wizard / 
Focus of Power / 
Unknown Perils is in play. 
set1-isengard.json
960 to 964      "phase": "skirmish", "condition": {"type": 
"perPhaseLimit", "limit": 3},
965 to 969      "cost": [{"type": 
"incrementPerPhaseLimit", "limit": 3},
Those 2 underlined parts are (even to my ignorant eyes) the obvious important ones for the limit (though caught my attention that 2 were used/needed for what seems the same task). One is before '"cost": [' (965) and the other after, but my point is that both come before '"effect": {' (974). 
Today I found the bug in duty, see my opponent's site 2 (with 5 available twilight, removed 3 and then the Slayer's skill stopped being an option, while 
Library of Orthanc still was):
http://www.gempukku.com/gemp-lotr/game.html?replayId=DurinsHeir$mf7a0cpfes9vkuw1