I've seen people claim that this faction is the most fun in the game, but I really don't understand at all what they're supposed to do, other than just die horribly.

I get that they're supposed to be the "weird units" faction, but those "weird units" are just blatantly worse than what the other factions can do.  Summon Familiar is great if you've got a ton of mana, worthless otherwise, and the game insists on always placing it at the very front of your formations in battle.  Spiders cost ridiculous mana, and really are worse than spearmen with leather armor; sure, web lets you cheese some fights, but those are mostly easily winnable fights; against anything with more than two unit groups, the Resoln army totally sucks.  Spiders perform much worse than trained troops against units that can actually fight, rather than being stuck in a web.  And then when you lose your army in a fight (since your army is complete crap, it's a lot more likely to happen to this faction), you can't actually rebuild it, since losing a fight usually goes along with spending all of your mana trying to win it.  Sure, Infection is nice, but you're relying on tactical spells to do more-or-less what the other factions do (and do better) for no mana, which means fewer and weaker strategic spells.

It seems to me that if you want a "magic-user" faction, Pariden is just a superior choice in every single regard.