Here’s a short background about Mars from Valve:
Abandoning the callous ways of his past and embracing a new identity unburdened by the legacies of his father’s archaic pantheon, Mars has turned over a new spear. Vowing to tear down the remnants of the savage old world to build a new empire, Mars is at last ready to assume the true mantle of leadership over gods and men, a position he now realizes has been wasted for eons by the petty whims of Zeus.
Mars is a melee, carry, initiator, disabler, and durable hero. His abilities are perfect for tanking and breaking enemy positions:
Spear of Mars – Mars throws his legendary Spear, damaging each enemy unit it strikes. The first enemy hero it hits is skewered and pushed back. If a skewered hero hits a tree, building, or cliff, they will be impaled to it and stunned.
God’s Rebuke – Mars smashes enemies in front of him with his shield, knocking them back and damaging them with a critical hit based on his attack. Imbued with True Strike, the assault adds bonus base damage when hitting heroes.
Bulwark – Mars wields his massive shield to block a portion of each physical attack when struck from the front or sides.
Arena of Blood – Mars summons an arena lined with the undead warriors of the Ash Legion, who block enemy attacks and movement. Enemy heroes near the inside edge of the arena will be attacked by spears, dealing damage and knocking them back.
Aside from the arrival of Mars, Valve also gave Drow Ranger a redesigned model.