The whole point of the runes is to acquire as much magic penetration as possible without sacrificing an item slot of which a better equipment could have been replaced with. With 20MagicPen from runes and 15 from Sorcerers boots and up to 25 more with wit's end your ultimate and hungering strike will destroy any person who has low magic resist, Blade of the ruined king and ravenous hydra are to provide some additional physical damage in case the target does have magic resist for more physical damage. Guardian Angel assures your fight participation to be more noticable as you can initiate fights, ganks, and bait abilities before even risking death.

STARTING ITEMS:

Hunters Machete + Health Potions + Sweeping Lens
Take Scrying Orb if you plan to steal enemy's blue/red buff. It will gain vision of target area in which allows you to smite and steal their buff.

KEY ITEMS TO BUY IN ORDER:

Stalker's Blade-Devourer (RUSH)
Boots of Speed
Wit's End
Sorcerer's Shoes
Blade of the Ruined King
Tiamat
Guardian Angel
Ravenous Hydra
Boot Enchantment-Furor (For sticking to targets)

ENDGAME BUILD:

Stalker's Blade-Devourer + Wit's End + Sorcerer's Shoes + Blade of the Ruined King + Ravenous Hydra + Guardian Angel

[Stalker's Blade - Devourer] Increases Ultimate damage with an unlimited scaling passive. Each large monster killed will yield 5 more magic damage when you channel your ultimate and provides a slow to stick with the target or deny them their escape with flash.

[Wit's End] Allows you to lowers enemy magic resist up to 25 and like Stalker's Blade-Devourer, will provide on-hit magic damage (42/Hit).

[Sorcerer's Shoes] Gives 15 more magic penetration, and with runes give you a total of 35! Imagine stealing 25 more with Wit's End, making your "Q" and ultimate much more painful.

[Blade of the Ruined King]
Besides the attack damage we use this item for its passive effect which does trigger 5 times during you ultimate and active which is great to stick to the target even more.

[Ravenous Hydra] Since our ultimate damage scales on attack damage it gives reason to get an item that also has a powerful active along with the AD, which provides some more physical damage so you may not be easily shutted down by magic resist (Not that they have much).

[Guardian Angel] If the team wants you to initiate teamfights and what-not take this. It will act as a tank item, else take The Bloodthirster for more AD and instane life steal (42% and if you use ultimate 72%)or Deathfire Grasp for magic damage amplification and stronger "Q"

WEAKNESSES:

-Early Attack Speed deficiency (Terrible jungle camp clear pre 6)
-Incomplete Build (Not very strong till Wit's End and Devourer are acquired)
-Ultimate Reliant (If your ultimate is countered you have to rely on your strong AutoA's)
-Lesser Tank (May lead to a very squishy team)

STRENGTHS:

-Good initiator with Guardian Angel
-Insane Magic damage output
-Loads of Attack damage and physical damage actives
-Strong fighter (Warwick is a fighter, due to prior misconceptions warwick is usually built tanky even if he has no tank qualities apart from his ultimate)