Rhyas is the kind of warlock that gets some flak on the forums. Realistically, not everything is created equally. This means some things are better than others, and Rhyas ends up on the lower end of the spectrum for Legion warlocks.
Being on the percieved 'lower end' doesn't mean Rhyas is bad. It means more that Rhyas is a bit more limited: she's got a handful of spells and nasty assassination potential, so she's kind of limited in what she can do.
Round 1 of Questions
1) How Vulnerable am I to Assassination?
Rhyas has the typical DEF16 and ARM14 you see in the smaller Legion warlocks. She's sitting on 16 health, which is about average. She's harder to hit, but not very hard to damage. This is kind of par for the course for warlocks.
Bottom line? Rhyas is hard to hit, but not hard to damage. You're likely better off buffing her defense, as DEF16 means that even boosted shots are probably rolling at 50/50. With Perfect Balance, the enemy can't try to use combined attacks to get around her DEF16, at least. However, she's still vulnerable to knockdown, which lets the enemy do fun stuff like slam things over her.
2) Can the warlock kill things?
MAT8 and a PS12 weaponmaster sword say that yes, Rhyas can kill things. Critical decapitation (double damage that exceeds ARM, cannot take Tough rolls) is icing on the cake.
Rhyas has SPD 7, and can get +1 from a spell. As such, she's got a threat range of 11.5 inches, and chances are that if an enemy caster gets within that threat range, you can kill them.
Note, though, that if you do plan on using Rhyas' offensive ability, make sure she kills the target. She has Sprint, so if you can hack at targets and kill them, you can promptly run out of combat and back to safety (kind of, anyway) and ignore free strikes thanks to Perfect Balance.
Better yet, thanks to Acrobatic, she can get through anything and make full use of her 10.5" base threat range. Screening models? She doesn't care, as long as she can put her base there.
Bottom line? If you're going to use Rhyas' melee potential, be careful about it. Note that she can only buy a limited number of attacks, given her FURY 5 and having a couple of upkeeps.
3) Spells/Utility
Rhyas does NOT have a lot of spells. She's got a whopping three spells, but part of that comes from having a whoppin' FURY of 5.
Rapport is a solid warbeast buff; it's a 2-fury upkeep buff that gives a warbeast in her battlegroup MAT8, RAT6 and the ability to transfer once without paying fury. Honestly, you kind of want a beast of a warbeast to take this spell, because it's her new best friend. You can also transfer to it even if you're outside of her whoppin' 10" control area.
Occultation is a situational buff that gives a friendly unit/model stealth. It's a nice 'screw you' to spells.
Dash is the one spell she has that's not an upkeep. Warrior models are immune to free strikes in her control area, and warrior models & Rhyas gain +1 SPD. This does two things: 1) increase the threat range on Legion infantry, and 2) pull your infantry out of a fight without risking death from free strikes.
Basically, Rhyas can make one infantry unit immune to direct fire, buff the accuracy of one beast, and increase the threat range of all of your infantry.
4) Feat
Her feat has the lovely title 'Tide of Blood.' Basically, Legion models get a free extra melee attack (YAY!) and the first time they damage an enemy model, you can place 'em in base contact with that model. You can use this for the obvious extra attacks, and you may be able to get a little extra threat range and positioning out of the feat, depending on what targets you go after.
I think you're more likely to use the feat to rip up the enemy army than anything else, but that's me. All else fails, you can use it to increase Rhyas' ability to get to the target.
Round 2 of Questioning
5) What kind of game does the warlock play?
Rhyas is light on fury and carries a small control range (all of 10 inches, as small as it gets for the game). As such, she's going to take the lead, and you may be wanting to take advantage of her personal melee capabilities. Make no mistake, if you CAN get to the enemy, then you can probably do some damage. However, you don't have the fury and durability to absorb much abuse, so you don't want to lean too much on Rhyas herself to kill things.
Any way you cut it, Rhyas WILL be close to the front just because she's FURY5. She's likely to throw out her upkeeps on turn one, and advance from there, looking for a good time to kill something herself.
The problem is that Rhyas has a fairly narrowly-defined list of stuff she can do: get infantry into the fight and make a beast accurate. She's very focused, and in that regard it's like taking a sniper rifle into an FPS game: you can do one thing REALLY WELL, but anyone that can get around that can give you some trouble. She's just not that versatile, as about the only thing she's good at is trying to kill things.
6) Win Condition
Ideally you'll use her feat to knock a chunk out of the enemy army. Otherwise, you're probably hoping to deliver the beast with Rapport on it to the enemy caster, as MAT8 will probably get through their defense without absolutely requiring the boost.
Really, Rhyas is about offense. Get the infantry in there, kill the enemy, and get something accurate to the enemy warlock. There's just not a lot of subtlty to her, or necessarily options. You can play the scenario, or go for the throat. She just seems a little more inclined to go for the throat.
7) Army Composition
Look to Rhyas' buffs to understand what kind of army she wants to run.
Rapport: bring a warbeast that's got both melee and ranged capabilities. Frankly, the Cariviean comes to mind; Rapport brings it to MAT8 for some punishing melee attacks and RAT6 on the spray is terrifying as well. I suppose you could put that on an Angelius, but it doesn't have the raw hitting power. The Ravagore has similar melee power, but I don't think it necessarily gets the same mileage out of the RAT6. Still, it's worth considering. The Scythean would lvoe the MAT8, but the RAT6 is wasted on it.
Occultation: this makes an infantry unit immune to direct fire, but doesn't do anything for its ability to survive blasts. If you were up for running two infantry units (which would maximize the utility of Dash) then you could go for Swordsmen (and give them occultation) and Hex Hunters. The Hunters go for infantry and the swordsmen go for harder targets. On the other hand, I kind of want to run Warmongers with them (or even Gatormen) as those guys are already resistant to blasts via 8 wounds + ARM16, and Stealth helps get around their lowish DEF12.
Dash: bring at least one infantry unit, and consider more. this makes you want to lean towards Faction models (sorry, Gatorman Posse...). If you want to roll something like Striders, this lets you throw Striders at lower-MAT enemies to try to slow them down, THEN extract them from combat and shoot the enemy in the face.
Past that, keep an eye on melee beasts for her feat, and potentially a Seraph for ranged support and Slipstream.
And, also, consider fury-management solos. You're FURY5. Never forget that. One full-out heavy beast rush can but you up to four fury. You're likely to have Occultation and Rapport out, which means you've got enough fury left to cast Dash and then transfer.
Conclusion
Rhyas is kind of limited, compared to other warlocks. If you want to run melee infantry, though, Rhyas is your wayt o do it for Legion. Like Lylyth, she's simple and elegant (and thus limited), but unlike pLylyth, I think she's a little more limited (Hey, Typhon triple-sprays and Seraphs can reach out and TOUCH someone...infantry may have trouble).