Isn't the artifact from Kind Idres priceless for the Sages? +1 Fight, +1 magic and -30% cooldown. The drawback is one relatively good fighter less in the beginning but anyway there are more then enough knights in the game (unlike the artifacts).
Or there is a similar artifact later in the game?
I'm still mostly in chapter 2, and while that artifact looks nice, it would be a tough call as to whether I'd swap out the other stuff my heroes are lugging around for it. Seems like there's a heap of tasty stuff to find.
potential spoilers? :
As for King Mark, the man's a beast! Give him some tough buddies (summerborn fits the bill for me at the moment), then sidhe roads him into the densest part of the enemy lineup (or on some annoying archers), hit breathstealer, hit cleave, and watch the limbs fly while he and his units continually regenerate. A plus to cleave weapon makes the carnage ridiculous. Say bye-bye to an enemy knight or two. Sidhe roads back out again when it gets too hairy or once cleave is off.
If Ywain is about with his soul mirror up, even better as then Mark won't be cleaved/dragon struck himself. Do a drive by or teleport over Ywain or another sage with spellweaver and a cavalry stack and suck up some free mana and maybe spam some spells (if Mark is mincing the archers). Meanwhile you're setting up your main army in a nice spot, or taking some VLs, while the bad guys are crowding around wiping bits of their friends off their clothes.
I found some giants that would be even more effective for this tactic, but they don't have the 50% reduction on sidhe roads, so he'd be stuck there manaless in the middle of the enemy army once his cleave/breathstealer band aid was finished. They do work a treat though when using the same tactic without the teleport surprise. Balor strike on top of the breathstealer/cleave combo makes everything else just explode into bloody goo. Maybe when he has a bit more mana I can make a teleport in teleport out breathstealing cleavebomb of death complete with giants. weeee.
Anyway - lots of ways to play this game, and ymmv, but I like a guy who can provide me with an alternative, or at least an adjunct, to the 'sit there in the woods and wait for the enemy with your firing then re-hiding archers strategy' against some of the hideously overpowered stacks you come across.