You need to first finish the war with Begus ie, (Make sure the Lich was summoned; happens during the cutscene, after capturing the capital). Then you need to start the war with Amagal, and during the attack on the capital, they will spawn below Dhranholl mine, and to the right of Meddling Castle. Afterwards, click on the group and you should be able to start a battle with them.
You need to be at least this far for them to spawn. If you are not seeing them, you might be in the post-war phase; you need to be in the war phase, not post-war. If they are still not showing up, and you are in the war phase, you might have ran into a glitch. P.S. If you have started the war with Amagal and they are not showing up, try waiting 1 turn. You can do that by having your soldiers advance until you spend all your AP for that round.
