This build is for tanky and crowd-controlling build for Hecarim. This build aims for being durable during early game and lane fights, then late game and doing some good damage by doing all this.

These runes are aimed for a tank-dmg Hecarim.

Greater Mark of Desolation: This aims to penetrate the early level armor of most champions to nearly zero. This helps pretty good during early lane combats.

Greater Glyph of Resilience: These runes can give real good flat armor for early game in lanes. It is for being durable while tower-diving and combats near minions. Considering that Spirit of the Dread requires as much as enemy around, this may be quite useful.

Greater Seal of Replenishment: Even if Hecarim has low mana costs, these runes help to manage your mana while spamming Rampage and use Spirit of the Dread when it is necessary during early lane fights with less problem with mana.

Greater Quintessence of Shielding: Magic resist per level quintessence aims to have a fair quantity of magic resist in late game. As personal opinion, basic attacks are generally used more than spells during early lane fights. So Magic resist per level quintessences may do more good for fights. Those may disagree can get Greater Quintessence of Warding for flat magic resistinstead of this one.

These masteries created for durability and damaging both. But as long as my aim is tanking, i go 21 points in Tank page and 9 points in Damage page. I gave points until i reach 10% armor penetration and gave ordinary tanking points in Tank page.

These items are aiming to deal some damage while tanking. Hecarims tanking is highly depend on the surrounding enemy suffering while Spirit of the Dread is on. In order to make them suffer more, you have to have more AoE damage, so starting with Sunfire Cape helps quite a bit. If you are satisfied with the amount of hp you have, you can go directly for Randuin's Omen, which can help with durability a lot, and with its active you can make enemy suffer more while Spirit of the Dread is on which is pretty beneficial when it comes to team fights.

After that, items like Trinity Force and Atma's Impaler can help for some damage to single target, assuming that you are gonna focus on the enemy nuker, whomever it is.

I put one more item on the item purchase part because player may decide how to go according to enemy. Mercury's Treads may be changed with Ninja Tabi, and any of the items above can be changed with Hextech Gunblade according to the players favor. I never had the chance to see how that works, but it seems to be a rational build worth to be tried for damaging.

There is a possible magic resist version of tanking, then Force of Nature and Maw of Malmortius can be chosen for both durability and damage.

In late game team fights;

Try to engage on the most squishy one the enemy has or the one needs to be disabled. Then use Spirit of the Dread in order to gain some significant durability because you will possibly be targeted after the use of your ultimate. Then use continuous Rampage on enemy. After the fight, chase the enemy with your Devastating Charge. By the way, i dont have any idea about how Devastating Charge+Onslaught of Shadows works. If Devastating Charge does not end, it will be good to use Devastating Charge before ultimate for extra damage.

Try to have heroes which has overpower AoE ultimates while there is Hecarim in your team. Fiddlesticks, Galio, Sejuani etc. which has AoE ultimates can disable and destroy everyone who have been caught when it is combined with Onslaught of Shadows. We tried the combo of Galio, Fiddlesticks, Sejuani, Hecarim and Vayne and result was awesome. when i dived with Onslaught of Shadows to 5 enemies, Galio taunted, Fiddlesticks "Crowstormed", Sejuani slowed and disabled, we killed them all with only the loss of half hp of mine and Galio's. So having these kinds of ultimates may be pretty beneficial.

Flash: Flash is for both chasing and retreating. While chasing, this can be needeed rarely because most of the people use flash to the other side of a wall, which is pretty annoying. in this situation, Devastating Charge may not be enough. For escaping, Hecarim is generally located at the real center of the team fights, so only way to escape if necessary is with flash. For this case, Devastating Charge's movement speed may not be enough for escape, so its good to have flash back in your sleeve.

Ignite: It is damage and heal reduction, nothing needed to be said ;)

Pros:

High health tank with considerable damage.
Extremely mobile, good at chasing.
Has considerable crowd-control abilities.
Does not have to go to base often.
Has showy skills which creates a natural "taunt"

Cons:

Weak against %health damages such as Vayne.
Needs to be backed up by players with brains, useful ones :D

These are my comeups about this champion. I hope you guys like it and enjoy playing Hecarim and other champions ofcourse. This is the Damage-Tank build for Hecarim, The Shadow of War. Ofcourse i will be waiting for your comments and advices. Have fun!