Grenade
From Exterminatus Manual
++Weapons++
The Fragmentation grenade (or Frag for short) is an ideal weapon against tightly packed enemies. Detonation causes the casing to fragment into hundreds of shards of flying shrapnel adding to its damage potential.
Tactics
Frag grenades are the only area of effect weapon the Imperium has in Exterminatus. Use them in pinch points to wound multiple Xenos at once. Grenades are also the marines only indirect weapon. Bounce them around corners or throw them over cover to hit targets beyond your direct weapons.