Those it descends on spiral toward a berserk state that'll eventually kill them. As well as the constant Red Thirst there's the rarer Black Rage.

The Blood Angels' curse is double-pronged. Sometimes a unit behind cover can shoot through it without penalty, but then at other times it blocks them and you'll try to shoot down from a gangplank only to hit the railing with every bullet. The other thing that discourages hanging back is that cover can be unpredictable. But activating overwatch costs momentum, which pushes you to be sparing with it and instead commit to risky advances, getting your commander with the jump pack and thunder hammer leaping into hand-to-hand with some giant monster that gives birth to swarms of smaller enemies or vomits acid. To emphasize that, it drops into slow-motion so you can see every shot go off and all the numbers flying out. It's tempting in games like this to go into overwatch on every turn, and overwatch can be powerful in Battlesector. My psyker, whose ability to create an image of a real scary face sounded kind of underwhelming, used an empowered version of that to drop six close-grouped tyranid units at once.

Its effect on some of the late-game skills is dramatic too. At first momentum doesn't seem like a big deal, but as armies get bigger there are more opportunities to rack up huge amounts over the course of a level.

Blood angels red thirst