Since his first sighting on the northwestern shores of Valoran, the towering, armored specter known as Hecarim struck a chilling fear into the hearts of all who laid eyes upon him. No one had ever seen anything like his titanic, ethereal form before, and the mystery of his sudden arrival was deeply unnerving. When Hecarim began to march eastwards, leaving a trail of desolate, lifeless ground behind him, the villagers of the plains fled their homes for the safety of nearby Demacia. In the city's now-overcrowded taverns, rumors about this inhuman phantom spread in hushed whispers. Some claimed that he was the vengeful shade of an ancient warrior, intent on destroying all living beings; one man insisted that they had seen him leading an entire legion of spectral cavalrymen; others still believed him to be the creation of some hateful necromancer. One Demacian commander, seeking to dispel the terror that had gripped the city, assembled a few of his finest soldiers and rode out to turn back or destroy Hecarim.
The commander led his soldiers to stand in Hecarim's path and braced for his assault. As the apparition bore down on them, an overwhelming sensation of dread gripped the warriors. The men, frozen in fear, could only scream as the ghostly titan ran them down, tearing them apart and trampling them beneath his iron hooves. Hecarim turned to the crippled, cowering commander and uttered a chilling statement: "This is merely the beginning. No mortal army can withstand the might of the Shadow Isles." With that, Hecarim departed, resuming his grim march. Driven to insanity by his nightmarish experience, the commander stumbled back to Demacia, where his dire warnings were dismissed as the ravings of a madman. While Hecarim's origin and intent were still a mystery, his destination became clear when he reached the Institute of War and, in a voice both ominous and commanding, demanded entry into the League of Legends.

"You don't understand... the shadows will consume us all..."
Former Demacian Commander

Warpath benefits from all movement speed bonuses, including those provided by boots, Ghost, and Devastating Charge.
Spirit of Dread restores health when nearby enemies take damage, including damage dealt by allies. Cast this during a large fight to maximize Hecarim's survivability.
Devastating Charge deals more damage based on the distance traveled. Try using Onslaught of Shadows or summoner spells such as Ghost or Flash to maximize this damage.
Slow effects significantly reduce Hecarim's damage output if you build him with high movement speed. Cleanse and tenacity help to combat this.
Activate Devastating Charge just before using Onslaught of Shadows in order to maximize damage output, since Devastating Charge gains bonus damage from the distance traveled during Onslaught of Shadows.
Hecarim can use Devasting Charge's knockback immediately after the forced flee from Onslaught of Shadows to knock an enemy even closer to or farther away from his team.
Hecarim can cast Rampage while moving, making it good for harassing melee champions, chasing enemies, and hit-and-run tactics.

Since Hecarim's passive, Warpath, grants him extra Attack Damage based on bonus movement speed, and Rampage as such a short cooldown, Trinity Force is an essential part of his build for both its movespeed bonus and sheen passive.
While defensive boots may be necessary for Hecarim in most games, being a melee AD character, in games where Hecarim can afford to build more offensively, his Warpath passive makes Boots of Swiftness stronger offensive options than Berserker's Greaves or Ionian Boots of Lucidity.
Note that Warpath scales off bonus movement speed. Buying Boots of Swiftness and stacking Zeals and Phantom Dancers does NOT provide the great tremendous damage that might otherwise be expected - at 18, with Boots of Swiftness and a full trio of Phantom Dancers, you can expect about 600 move speed, which will yield (600-320)*.25 = +70 damage from Warpath, instead of 600*.25 = 150.