I was playing MP with a friend as Svart and while I agree Domination is awesome, The mistform was completely harmless.
They are not for attacking; they are for pillaging. Even without open borders, 4 archmages summoning mists (at the border) will destroy 4 towns/etc! If you have open borders and can enter someone's (or the AI's) civ, THEN cast... well, the whole place goes to crap in about 4 turns and you become very rich.
In the absence of the ability to pillage (via friendly territory and/or war), they are illusions and as such do not kill the unit attacked. This is quite nice when you would like to p-level weak units (need an archmage replacement? Send a mist summoner with an adept to some barbs/animals).
Basically, they are summonable HN Shadows that do not have marksman, but can pillage and do not kill.
+ unit graphic = pwnrshp
In PvP, I think summoner trait might help. That way, the player would need to create hawks (if they still see invis), spot the mist, devote units to kill it or it continues to wreak havoc on their improvements for 2 more turns. With svalt, you get raider, so a 4-mist pillage turn = ~400 gold. Make sure you are running a cash-rush civic for cheater synergy. The 4 arches are like a reusable great merchant that destroys all nearby improvements (with raider). If your neighbor (AI) has a huge civ and will not open borders, you can cast worldspell and bring the whole crew (with defenders) into their civ to damage deep and complete - without declaring war.
I give my svalt arches: mind 3, enhancement 2, shadow 3. I wait for arcane lore to promote enhancement 2 so as not to waste a promotion. My accompanying mages (2-3) have air2, shadow 1 and 2, mind 1, nature 1 and 2 (1st node = air, if 2nd node = body).
We seemed to have skipped from basic civ-use/strat into complex world-crunching advanced strat but that's ok.
Someone might recommend the OP some starter-esque strats?
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Xienwolf: Feel free to cut/paste and edit any of my posts (and anyone else's - I hereby give you permission) into your manual for the strat section.