Glad to see a Roman mod that finally includes the Parthians! I've always dreamed of being a glorious cataphract charging into battle, and it would appear that the Rome at War mod includes two-handed spear use from horseback:
Which for me is a dream come true.
The highest-tier cataphracts ought to have very heavy armor both for themselves and for their powerful steeds, and should be
very hard to kill. Several powerful close-range javelin hits or forceful spear thrusts could bring them down, as could mobbing them with average infantry, but the odd arrow headshot or sword slash should barely faze them. As for weapons, their primary arm would be the
kontos, or "barge pole" as it was called by the Greeks, a thick, heavy, long lance. The cataphracts would wield it with two hands, preventing the use of a shield, which isn't so bad since they're covered in heavy armor anyway. The
kontos could have a high STR requirement and would have a fairly slow attack speed, but it would hit with a lot of power and would crush through blocks. That way, a hit from a heavy two-handed lance wielded by a strong man charging on horseback could not be stopped by a little gladius parrying it, or by some shield. The downside is that, as the
kontos is long and slow, it's not any good at close quarters.
I've always badly wanted to play Mount & Blade as a heavy cataphract impaling foes with a furious charge with a two-handed lance, and now that dream finally looks like it's in sight! Good luck with your mod!