From Lexicanum

Tyrannic War Veterans are an elite section of Ultramarines originally created under the watchful eyes of Chaplain Cassius. These veterans are made up of the warriors who have fought and survived battles against Tyranid forces since the attack of Hive Fleet Behemoth. Since then the Ultramarines have spared others the difficulty of fighting the Tyranids while constantly rebuilding their First Company with these specialists. Over half the first company is now made up of Tyrannic War Veterans.

Tyrannic War Veterans are trained to use krak grenades as a special close combat weapon when facing Tyranids. They also use special Hellfire Ammunition in their heavy bolters.

Source

• Codex: Space Marines (4th Edition)

Space Marine Infantry
Command	Master • Captain • Master of Sanctity • Reclusiarch • Epistolary • Codicier Command Squad • Apothecary • Company Standard Bearer • Company Champion
Standard	Tactical • Assault • Devastator • Scout • Veteran • Tyrannic War Veterans • Honour Guard
Terminators	Terminator Squad • Terminator Assault Squad • Terminator Command Squad