Faction traits and enemy skills

Quick answer

Tyranid Synapse and the Ork Waaagh! are confirmed faction traits. Skaven, Khorne and Gloomspite Gitz have named skills.

Faction traits sit on top of the usual horde. Auroch's 17 September post gives two worked examples and says the rest arrive with the game. Enemy skills, from 3 September, are the individual units that charge, box you in, or poison you.

Faction	Trait	What it does
Tyranids	Synapse	A critical bioform, Hive Tyrant used as the example, puts nearby Termagants and Hormagaunts into Berserk. Kill it and they drop into a short stunned shock.
Orks	Waaagh	The screen goes red, more Orks spawn, the ones already there frenzy.

Unit	Skill
Von Ryan's Leaper	Rears up and pounces over the crowd
Bloodcrushers	A telegraphed charge you can leave, or stop
Ratling Gunner	Crosses the screen and leaves a hazard lane
Bell of Doom	Nearby Skaven gain Warded, so weak rats get tougher
Acolyte Globadiers	Toxic globes that poison you

The 6 October post adds Gloomspite Gitz: Bad Moon, grot legions, Squigs, Troggoths. Their skills are not written out. The version that puts these in the order you meet them is how a run actually goes.

Horde pressure in an official store screenshot