Thrax was told to use a glaive instead of a spear for one main reason: the spear was made more for piercing attack, as such, it could not fully make use of his explosive strength. The glaive was a larger polearm-type weapon that would allow Thrax to still use most of his skills and retain his preference for polearms, thus letting him use both of his greatest proficiency.
[Power Glaives - Tier 4]
[Polearm]
[Length 3.2 meters, weight 46 kilogram]
[Increase strength]
The glaive he used was long and heavy, it was topped with a large one edge blade. As long as the wielder had the proper strength to swing it around, it was the perfect weapon to use when fighting against a group of enemies.
Swisshhh! Swisshh!!
Thrax was able to use his overwhelming strength to swing the weapon easily to tear his opponent apart.
For those who read Doomsday Pillars, I hope this explains. Thank you for all the support, much appreciated