Dahlia and Skarath are one of the few and proud merc/minion choices available to the Retribution. The Iosans have the distinction of being the only Warmachine faction that she'll work for.
They're a well-defended, lower-armor duo that mixes denial (from the elf dame) and a nasty offensive (the big freakin' snake). As they are a Warbeast & Lesser Warlock, the big selling point is that it's a jack equivalent that does NOT tax your caster's focus and does NOT absolutely positively have to be in their control area. It's kind of like a Jack Marshal and warjack in one, except that if they kill the marshal you flat-out lose the jack.
It's 9 points for the pair, on par with a heavy warjack. If you lose Dahlia first, you lose Skarath. If you lose Skarath, Dahlia runs out of Fury pretty fast and then either cuts herself or...stands there and takes up space, perhaps on an objective.
SPD 6 is respectable enough. She has a hell of a defense with DEF 16, but ARM 12 is squishy even with 8 health boxes. Her other basic stats are pretty irrelevant as she has no weapons. I mean, her sculpt has a friggin' flute. You were expecting?
The important stuff for Dahlia: FURY 4. She has two spells, and both cost 2 fury. She's got enough to pop both, or double up on Mist Walker. Additionally, she's got Skarath's animus, which CAN impact friendly models and costs 1 fury.
Haunting Melody is a RNG Control spell. Enemy living models cannot give/take orders, and cannot make melee or ranged attacks at Dahlia. Apprently she's good with that flute. It's the close-range don't-kill-me spell. Not bad, as it means the enemy will have to devote long-ranged guns to kill her if she has this up. (and of course she can do transfers, so yeah...)
Mist Walker is RNG 6; but can only impact Dahlia or Skarath. The target gets Pathfinder and Prowl, so when in concealment/cover they get Stealth. It gives me this ridiculous image of a huge freakin' snake trying to hide behind an inch-high wall, and not really succeeding.
The special rule 'Charmer' means that when a firendly warbeast frenzies in her control area (read: Skarath for this post) that the beast can forfeit its activation instead. Important, because it means Skarath doesn't turn around and shank HER, because he'd almost certainly kill her.
Lesser Warlock is essentially the warlock-lite set of rules, and Limited Battlegroup means only Skarath can be with her. Frankly, with Fury 4 she has no business trying to run more than one beast anyway.
Skarath is a glass cannon. SPD 6 is nice (and Mist Walker helps out), and DEF 14 is reasonably solid for defense. ARM 16 is...not so solid on a heavy. Skarath has about 25 wounds on his spiral, though he's a little light on Mind. He's harder to hit than our myrmidons, but has all the armor of a light.
Really, there's one big draw for Skarath: SP 10, POW 12 corrosion with continuous corrosion. With FURY 4, he can boost to hit the important stuff, though with RAT: crap you'll probably NEED the boosts. Since the only ranged Retribution option that ignores stealth so far is the Stormfall Archers (...and soon to be Discordia) this is something of a selling point. The long range may set you up to catch a few targets with an aim bonus, though you have to ask your enemy nicely for it.
Skarath's no slouch in melee, either. He's MAT 6 with a PS16 Reach attack. Critical Consume means that if you hit a small based non-warlock/caster model it's removed from play. Remember to make 'om-nom-nom' sounds when this happens. With PS16, there's a good chance you'd out-and-out nuke anything short of a Lesser Warbeast in one hit (and losing a Shredder to a crit consume would hurt, trust me...) it's also nice to have a Remove-From-Play option available, even if it's not the most reliable of things.
Being a snake, Skarath gets the Serpentine rule, which means he's immune to knockdown, but incapable of slams/tramples.
His Animus is a RNG 6 1-fury spell that grants a model Riposte. Riposte means that when the enemy misses the target model, that model gets to take a swing at them immediately, then the animus expires. Funny story, it's called Serpent Strike.
Skarath also has a neato warbeast bond, so he can CHOOSE his frenzy target. A fully boosted MAT 6 POW 16 hit is nothing to sneeze at. If the dice go south, this is nice. Otherwise, you're probably trying to avoid losing Dahlia's fury battery and the ability to really CHOOSE what you're doing with a heavy hitter in your army.
On the Field
The first turn or two will either see them both running (and Dahlia discarding a fury so she can leach the forced fury off of Skarath) or Dahlia casting Mist Walker 1-2 times and riling Skarath to refuel.
After that, it's probably time to look for a good spray shot. Even if POW 12 won't kill 'em reliably (IE: shield-walling guys or, oh, ARM19 Dawnguard for example...) the corrosion can still do the deed.
MAT 6 is passable, but it also means Skarath is capable of handling itself in melee.
If you can find something that a couple POW 16s will cripple/kill (or do something prevent boosting to-hit rolls, like a Disruptor Bolt or eEiryss) then you've got yourself a melee target. Note, though, that Skarath's ARM 16 is NOT gonna stop heavy hitters from mincing him.
In your Army
As D&S are a 9pt unit, you're probably not taking them in 15 points. At 25, it's possible to include them, but they're still a chunk and you probably want at least one myrmidon as a heavy hitter. Don't get me wrong; the flutist and snake are versatile and all, but you have warjack points to spend first.
I'd probably consider them seriously starting at 35 points.
When it comes to capabilities, I think three things make them stick out and should be considered when deciding whether or not to take them.
1) Do you want a SP10, POW12 Continuous Corrosion shot that can get there without too much trouble?
2) For focus-greedy casters, it's a heavy that does not require focus to operate.
3) It's a heavy that does NOT have to stay in range of your warcaster, but still gets to boost more than once a turn. Helloo, flanker.
There are two major cons that come to mind with this pair: Durability and the inherent weakness of a lesser warlock.
Dahlia and Skarath both have solid defense values (16 and 14 respectively) but sub-par armor (12 and 16 respectively). They have a reasonable number of health boxes, but when your ARM is like that...there's room to worry. It's just something you have to keep in mind when it comes to AoEs and accurate heavy hitters.
As for lesser warlocks/beast buddies? Losing one part of the pair is essentially losing both. If the enemy drops Dahlia, Skarath leaves. If the enemy drops Skarath, Dahlia loses her only Fury battery.
Interaction with Retribution Warcasters
Kaelyssa can use Banishing Ward with them. Since they can get Stealth without too much trouble, her feat is unlikely to leave them out to dry as the only non-stealthed targets.
Rahn can help them out with Force Field (...since boosted AOE hits are a worry with Dahlia's mighty ARM 12, which means it's transfer damage to Skarath...) and can always use Telekineses to help one or both of them out of a jam.
Epic Eiryss can rob a jack of its ability to boost, which works nicely with the solid natural DEF of our intrepid duo.
Anything past that? It's a matter of army composition and how much you want to save focus on your caster.
Dahlia and Skarath are, I think, viable options for the Retribution. They're one of two sources of sprays at this point in time. They can operate independently of the warcaster (though this also means most of the buffs don't work; see this discussion on Cylena & co for expansion on that) in terms of resources and control distance.
So long as you bear in mind their relative fragility and don't try to take them in smaller points levels, I think you can expect mileage out of them.
Additionally, I think they offer a couple of 'soft' benefits to the Retribution player:
1) It's a break from painting armor and myrmidons
2) If you haven't played Hordes, playing with the Fury mechanic will be a good learning experience.
Give 'em a shot. They're perfectly workable in a Retribution army, and if you have a Circle or Troll army, they can pull double-duty for you there.