Which series in the Heroes Chronicles did you like the most?

I can't complete the sixth mission in Elemental Guardians, "The Magic That Unites." It seems the objective is to gather creatures, but it doesn't specify which ones. The mission description says to find three magical elementals called "The Firsts." I've destroyed all the enemies on the map, and I have plenty of different creatures, but I don't know what to do next. Help! :D