First of all a definition: A somethingstar is a combo that's nearly impossible to kill, dishes out damage like nothing else and costs a fortune, similar to the deathstar, so it is only seen in high- to midlevel point games.
A ScreamerStar comes from the Chaos Demon Codex, featuring four Heralds and nine Screamers of Tzeentch. First of all enter the Screamers: Fast as a hurricane, no ranged attack, normal close combat attacks an a pseudo melta bomb attack, counting as jetbikes. These little guys are the armor and wounds of the Screamerstar.
Now the Heralds of Tzeentch are the real punch of this thing, four buggers on discs, all fitting in one HQ-slot. Heralds are level 3 psykers with access to divination and change, so an Exalted Locus of Conjuration makes sense on one of them. Two of them get Exalted Rewards, the other ones Lesser Rewards. Doesn't sound like much, does it?
The armor comes from one Psychic power named 'Forewarning' and the Exalted Reward 'Grimoire of True Names'. Forewarning gives the unit the psyker is in a 4++ save. The Grimoire of True Names make this a 2++ save on a dice roll of 3+, and all demons of Tzeentch reroll failed saves. One of those heralds will likely have forewarning, normally the player rolls eight times on the Divination table and the last four times he invests in 'Flickering Fire'.
The other Exalted Reward could be a Portaglyph, the two Lesser can be what the player likes. Now
this unit has a body count of 13, after the first turn normally a rerollable Invulnerable Save of 2++, and up to 16D6 S6 AP4 witchfire hits on 24" which are likely ignoring cover. Models who were hurt have to make a toughness test. On a fail they take D3 additional wounds with no saves allowed.
The Fateweaver is a common addition as warlord, as he allows one dice per turn to be rerolled. Guess which.
Counters: There are some weaknesses. First of all, it is expensive and, thanks to the Emprah, not a pointing unit. It depends heavily on Forewarning, so if the munchkin doesn't get this, the unit has lost its main defence. The Grimoire carrier is vulnerable to Barrage Sniping, the whole unit to Mind Strike Missiles. The Unit has trouble to kill a few things: AV 12 flyers, everything AV 13 or higher, tough MC and large TEQ Squads. Also a good counter: Bring an everlasting Runepriest to the date.