"We will crush all those who dare resist the will of the Red Army! Abandon your posts... Abandon your homes... Abandon all hope!"
- – Commissar Markhov, Call of Duty: World at War
Imagine a Commissar. Scary guy, big sword, nice hat. Now imagine a full regiment of Commissars, all with their peaked caps shadowing their face as they all snarl and raise their lasguns at the same time. I don't think you'll need to imagine shitting your pants. You certainly won't have trouble picturing the Terrax Guard.
Hosting a huge Schola Progenium facility called the Schola Excubitos, Terrax ensures a system of elite and disciplined training and fancy uniforms to a similar extent to the Mordian Iron Guard. While it's hard to speculate on what additional training they receive, it's probably likely that they practice their markmanship by firing at a range of five centimeters with a bolt pistol at “volunteers” from their penal legions.
Also notable for preferring to use meltaguns as squad special weapons and lascannons as support weapons. Which makes sense as their carapace armor means your anti-infantry weapons won't do much, meaning vehicles are the only real threat to them and besides that the lascannons can waste any enemy anti-infantry support weapons quickly.
They were present during the Macharian Crusade, but took significant damage from friendly fire in the form of both the Star Phantoms and Marines Malevolent - the former of which was declared "not heresy," but did sideline both groups from further active duty in the crusade.