Well im new to lotr sbg and I just brought army of the dead with a banner bearer and a king of the dead. I have a cheap Aragorn and i would like to use him aswell. So who can i put with my army of the dead + Aragorn ?

If I assume your king of the dead will lead your army of the dead I would look for troops for aragorn to lead.
Is it aragorn king or aragorn strider?
If it is strider I would look at arnor (rangers, dunedain, warriors of arnor) or other fellowship WINNERS (ie boromir, legolas, gimli, gandalf)

Yup, Aragorn, Legolas, Gimli, Grey Company, Halbarad, Elodan and Elrohir would be cool plus fiefdom warriors or Minas tirith warriors used as warriors of the southern gondorian lands.
Makes the army fit with the battle at Pelagar.

Don't forget you can convert a warrior of the Dead to resemble a Cpt and use the Captain of Dol Amroth stat line.

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