option 1 (scout unit, used with extreme caution, graphics should look like a sneaky guy with a cloak):

１　ｄａｒｋｌｅｇｉｏｎ　１X 3 flux

0 attack, 2 health, 3 movement, 4 or 5 sight 0 range
ability:　Ｉｆ　ａｔ　ｔｈｅ　ｅｎｄ　ｏｆ　ａ　ｒｏｕｎｄ　Ｓｔｅａｌｔｈ　ｍａｓｔｅｒ　ｉｓ　ｕｎｄｉｍｍｅｄ　ｉｔ　ｂｅｃｏｍｅｓ　ｓｔｅａｌｔｈｅｄ．Ｗｈｅｎ　ｓｔｅａｌｔｈｅｄ　ｔａｒｇｅｔ　ｐｌａｙｅｒ　ｃａｎ＇ｔ　ｖｉｅｗ　ｉｔ　ｅｖｅｎ　ｉｎ　ｓｉｇｈｔ　ｏｆ　ｔｈｅｉｒ　ｂｕｉｌｄｉｎｇｓ　ｏｒ　ｃｒｅａｔｕｒｅｓ．　Ｓｔｅａｌｔｈ　ｍａｓｔｅｒ　ｌｏｓｅｓ　ｓｔｅａｌｔｈ　ｉｆ　ｉｔ　ｍｏｖｅｓ　ｏｒ　ｗｈｅｎ　ａ　ｃｒｅａｔｕｒｅ　ｎｏｔ　ｏｗｎｅｄ　ｂｙ　ｔｈｅ　ｓａｍｅ　ｃｏｎｔｒｏｌｌｅｒ　ｔｒｉｅｓ　ｔｏ　ｍｏｖｅ　ｉｎｔｏ　ｉｔｓ　ｓｐａｃｅ．
(great for spying on enemy's territory. Not really good at anything else. You're paying flux for information basically. also it takes a full round for it to stealth so it takes skill to get him into position... 4 sight to make him weak, 5 sight to make him worthwhile to play in most darklegion decks. cost and usefulness on par with a hell kitty :3)

option 2 (a sort of stealthy rogue who uses crossbows)

1 Darklegion 1X 5 flux

1 attack, 3 health, 4 movement, 4 sight, 2 range
ability:　Ｉｆ　ａｔ　ｔｈｅ　ｅｎｄ　ｏｆ　ａ　ｒｏｕｎｄ　Ｓｔｅａｌｔｈ　ｍａｓｔｅｒ　ｉｓ　ｕｎｄｉｍｍｅｄ　ｉｔ　ｂｅｃｏｍｅｓ　ｓｔｅａｌｔｈｅｄ．Ｗｈｅｎ　ｓｔｅａｌｔｈｅｄ　ｔａｒｇｅｔ　ｐｌａｙｅｒ　ｃａｎ＇ｔ　ｖｉｅｗ　ｉｔ　ｅｖｅｎ　ｉｎ　ｓｉｇｈｔ　ｏｆ　ｔｈｅｉｒ　ｂｕｉｌｄｉｎｇｓ　ｏｒ　ｃｒｅａｔｕｒｅｓ．　Ｓｔｅａｌｔｈ　ｍａｓｔｅｒ　ｌｏｓｅｓ　ｓｔｅａｌｔｈ　ｉｆ　ｉｔ　ｍｏｖｅｓ　ｏｒ　ｗｈｅｎ　ａ　ｃｒｅａｔｕｒｅ　ｎｏｔ　ｏｗｎｅｄ　ｂｙ　ｔｈｅ　ｓａｍｅ　ｃｏｎｔｒｏｌｌｅｒ　ｔｒｉｅｓ　ｔｏ　ｍｏｖｅ　ｉｎｔｏ　ｉｔｓ　ｓｐａｃｅ．

(great for both spying and for one shotting dimmed 1 health creatures who unwittingly get near him. This version would help darklegion out with its lack of ranged creatures. cost and usefulness on par with an astridian diplomat.)

Option 3(loner who likes his privacy):

2 DarkLegion 1X 5 flux
Isolation 1

3 damage 7 health 3 movement 5 sight 1 range

Ability (Different ability than options 1-2!!): All figures within sight gets their sight reduced by 1. (including owner's figures! This makes it more fair, basically on par with a journeyman necromancer or yalrinian fiend. Alternatively it could have 7 flux as a cost and be without Isolation 1... )

Option 4 (Endgame bringer of pain, basically master of assassins, graphics could be either a cloaked guy with poison tipped crossbow or throwing knives):

3 darkLegion 7 Flux
poison 1 (if this creature attacks another creature, the targeted creature takes 1 point of damage at the end of each round)

2 damage 6 health 3 movement 4 sight 4 range

Ability (Different ability than options 1-2!!): All figures not controlled by the player who controls stealth master loses 1 range of vision if in stealth master's sight. Cost and usefulness is around the same as agent of death and is as hard to bring out.)
DarkLegion really needs another ranged unit or two ESPECIALLY LAND UNITS WITH RANGE!! Having just octopus and ghost ship which are both weaker than their sylvan counterparts, driftwood and the other ship makes pure darklegions decks the worst in terms of ranged. Having to use lysis on thistle falcons is just sad.

Another possibility for his ability could be:

Hide presence: X target creature is hidden from other players' sight until end of round. X is the cost for each space the creature moves within the sight of other players.

The way it will work is if a player has an Ivory spire out and a tenderfoot archer near it, and the owner of stealth master would really like to destroy the spire but don't want to lose a unit, he'd pay 5 flux to stealth a hell cat for example, and then surge the cat to go and kill the spire. 5 because the cat will need to move 4 spaces within enemy sight and the extra flux is to cover for the attack. However if for example the tenderfoot is near the edge of the tower's vision, and thereby is extending the sylvan player's vision and the owner of stealthmaster forgets this and only used 5 flux but the vision of the other player was 5, the cat would not be revealed to the enemy unless it attacks the spire or move more than 5 spaces. Also instead of it being shrouded in fog, whatever is being stealthed can move in revealed spaces without being seen. If the stealth master owner paid enough flux then the cat would not be seen but the owner of the spire would still see the revealed spaces of the tower, just not the cat, so he has no idea where the cat is whereas a moving unrevealed space is a dead giveaway. HOWEVER, if the cat was stealthed enough for example by this ability to remain hidden for the round, but the owner of the spell has 2 tenderfoots, then if one tenderfoot tried to move to the space where the hell cat is, then the tenderfoot stops next to the hellcat and is dimmed. The hell cat is then revealed and the other tenderfoot gains the opportunity to attack the hell cat. If there's only one tenderfoot, it just dims next to the cat as finding the cat was a surprise for him as his original intent was to move to the cat's space, and in effect the cat is safe until the next round begins anyway. Ironically this provides a dilemma for the player with the spire. If the first tenderfoot moved to a space near the tower where he thinks the hellcat is when in fact the hellcat was in another space, the hellcat is hidden and is safe even if the other tenderfoot finds him this round. because the owner of the cat would move first next round and be able to get the cat out of harm's way. Also the ability should be limited to one creature per round as it would take all the stealth masters concentration to stealth a creature and it should also dim after using this ability.

In conclusion this version of stealthmaster's ability would allow for a player to attack buildings without retribution. Though the problem would be mountainfolk players have attrition so stealth would only be worthwhile on non mountainfolk players or if stealthing a creature that has enough HP to survive attrition and move out of harms way the next round. OR the stealthmaster could stealth a unit that the player doesn't want destroyed using one flux. As long as it doesn't move or get discovered by an enemy creature it will remain hidden.

I think it's debatable whether this version of stealth master's ability should be allowed for stealth attacking an enemy creature... remember, the player has to pay 1 extra flux to remain stealthed for it's attack, so it is fair flux wise...