Complex sentence: Tydeus kills all the ambushers except Maeon so he can carry the news back to Eteocles, and then attaches the battle trophies to an oak tree as he prays to Minerva.
Split into simpler sentences: Tydeus kills all the ambushers except Maeon so he can carry the news back to Eteocles. Tydeus then attaches the battle trophies--taken from the slain--to an oak tree as he prays to Minerva.