Sion and Undying Curse - Undead
Personally i find Sion one of the most useless units for Empire. Only reason i train them is to make them into Governors for party bonuses or sometimes Defenders for party bonuses too. But leveling them for those traits is also ridiculously hard as range units eat them for breakfast. They are significantly inferior to 6 man units, Sovereigns, Champions and even the Henchmen. Honestly i dunno why they are even exist in this game.
That is, until Undead Empire...
For Undead Empire, Sion with Undying Curse does wonders. You can turn the Sion into a full fledged 6 man unit AND they will AUTO-REINFORCE the numbers if any died, over time, provided you have researched the required tech for 4, 5, 6 man requisition. Suddenly Sions are one of the strongest units in game for Undead only. Classes such as Assassin, Defender, Warrior and Governor are now much more viable to spec the Sion. Leveling them is so much easier too because Undead have Terror to lock all enemy units while your Sions slowly last hit every enemy and increase their unit count. Mage spec still useless though lol.
I dunno if this is intended... but im sure am enjoying it... If only we can have Sions with Skeleton/Banshee skins...