A cool thing I discovered a while back

[ExisT]

A while ago I discovered that if you have a class with a "charm" or "conversion" spell(like Voice of El) you can use that ability to charm an opponents commander. If you get lucky, you might charm a commander that get's you access to that factions resources and possibly their rituals/units.

In my situation I converted the Illusionist as the Voice of El. I don't remember if I could move him around like a normal commander or not, but him being on my side granted me access to his gems and I could recruit his illusion mirrors in my castle. I thought that was pretty cool. I didn't get far enough in that game to really exploit it(I died shortly after) but I can only wonder what the possibilities could be if you were lucky enough to charm a commander like that and exploit it throughout the game.

[Arsonium]

Charmed commanders are like all other units assigned under their charmer, you'll have to unassign him through Transfer units before being able to move on map, cast rituals and so on.

[Entropino]

I once got a dwarven egg carier(forgot name) as an illusionist(confuse)

[Orpheus]

This works with Charm, Confusion and Conversion abilities. It does NOT work with Enslave, which will 'de-commanderize' units affected by it. Also, while you may encounter neutral Captains, Cultists etc guarding various sites, they may not be able to act as comanders even if charmed. If the character has an individual name (eg "Bob the High Lord"), s/he is a commander, able to move independently, lead units and potentially may have rituals. Nameless units ("High Lord") just act as any other unit. The Enslave effect removes a unit's name, hence converting commanders into normal units.
There was a bug at one point, which may or may not have been fixed, where the Reanimate weapon effect would preserve a unit's commander status. So casting Bolt of Unlife on Bob the High Lord would result in Bob the Soulless, who has the same stats as any other soulless and no rituals, but is able to lead units like any other commander.

[Chyre]

This is a bit of a guilty pleasure for me; I like to play as witch or dryad queen, treating Conquest of Elysium as a game of Pokemon. Gotta catch 'em all!

[Orpheus]

I'm glad it's not just me. High Cultist can do it too (Confusion works), as can Demonologist once you start getting Succubi. Voice of El is probably the funniest, recruiting necromancers and demon-worshippers into God's Holy Army, where they continue raising demons and undead.

[Chyre]

I can top that. If you cast "Curse of the Frog Prince" on an enemy commander and then subsequently charm him, you end up with a FROG COMMANDER!

[Hand's Hermit Permit]

Frog prince confirmed.

[Gilmoy]

Hah! My Illusionist captured the Baron! I gave him an Elven Cloak and had him run around alone to flag forests. I didn't get any of his skills, but it was fun to read his description!

[Jakalor]

One thing to note, apparently Ichtyids count as animals and the Ichtyid Captains are counted as commanders when charmed. Also, Hellbind Heart keeps commander status.

[oort object]

fun fact: when a leader is murdered by an effect that transforms any living thing it kills into a soulless, they still count as a leader. (they lose all of their spells and rituals they had in life tho)