Hi,

Say if I have a Lothlorien army and want to ally Elrond.
Can Elrond command Lothlorien warband? Or can he only command some allied Rivendell units?

Hi - Elrond would have to command an allied Rivendell warband. Or alternatively, you could just ally him on his own with no warband. But he cannot lead Lothlorien troops.

A hero can only lead troops from their own faction.

Hope this helps

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T'was in the darkest depths of Mordor, I met a girl so fair..
But Gollum, and the evil one crept up and slipped away with her..

Keep in mind many Rivendell and Lothorien troops are almost identical. Have him lead your warrior base, for instance.

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By all that you hold dear on this good Earth,
I bid you stand, men of the West!