The Screamerstar is a Warhammer 40,000 tactic using 7th Edition Chaos Daemons. It is made of rerollable 2++ and FUUUU-. When used right it can be a devastating move (and a major annoyance) to the enemy player, but will quickly lose you friends.
Has absolutely nothing to do with Starscream, despite being just as annoying (Wanna' bet?)
How to Lose Friends and cause Heart Attacks
Basically you take some heralds of Tzeentch on discs, roll on divination and try to get forewarning, as well as taking the Grimoire of True Names and preferably Fateweaver for his rerolls. Once you have a herald with forewarning you cast it on a unit of Screamers (usually 6-9) and grant them a 4+ invulnerable save. The grimoire then brings it up to a 2++. Since they are Tzeentch units they reroll saving throws of 1, giving you a crazy death star that can only be killed by convoluted or rare means. Keep in mind the herald on the disc can still be killed, so remember to roll for Look Out Sir! for those precision shots and do not keep him in the front of the unit.
The unit can dump a large number of attacks at either range or melee, with enemies up close getting to suck on a large number of lamprey bites and greater etherblade swings while enemies from afar get to chew on flickering fire boosted to warp charge 3. Thus your enemy can't really ignore it because not only is it absurdly durable, but it's also extremely killy against just about any target. For added rage inducement, someone can throw endurance on it, and in rolling for forewarning you'll probably have a bunch of other goodies from divinitation to throw around buffs and debuffs like an asshole.
You can try to tarpit them, although given the fact that screamers have access to AP2 attacks and a good number of attacks normally, you'll need something purpose-built to last, like Azrael inside a 50-man guard blob. Shadow in the Warp, Rune Staffs, and other anti-psyker gear also helps a lot here. If your army lacks both of those, killing Fateweaver and his rerolling bullshit helps lower the reliability of the Grimoire.
There are a couple other ways to lessen the effectiveness of this unit. The weakest one is to use something that forces rerolls of successful invulnerable saves; as a die can only be re-rolled once, it makes a slight dent in the unit's effectiveness, although not by much. The second method is to bring Strength D weapons, from Lords of War or an allied Knight allied detachment's melee, drowning out his cheese with your own. The third, optimal way is to grab your opponent and beat his brains into paste for being a cheese-mongering cunt.