Twilight or black dragons? Probably black dragons around the 3rd or 4th month. It's a shame the question is so specific; the most powerful unit is, of course, the lava dragon. However, with the release of the second add-on, when I hope Axel's bonuses will also be given to the Knights of Isabel, they will be a very formidable force around the 3rd or 4th month. With a speed aura on top of that, they will be able to completely or almost completely destroy two enemy units at the beginning of the battle, and with the support of archangels... just let Haven develop:D