Undying is a melee strength hero who thrives off of his staying power in fights, with his abilities growing stronger the more heroes and units there are involved in any given engagement.
He is typically played as a greedier, 4th position support/utility hero.
Skills
Decay is always skilled first but usually maxed last.
Soul Rip is often skilled and maxed second.
Tombstone is skilled last but always maxed first.
Flesh Golem is usually skilled first around level 9, then always skilled thereafter.
Items
Starting Items: Courier and/or Observer Wards, Tangos, Healing Salve, 3x Iron Branches, Clarity.
Early Game: Flying Courier, Observer Wards, Boods of Speed, Magic Wand.
Core Items: Arcane Boots, Mekanism.
Luxury Items: Vladmir's Offering, Drums of Endurance, Pipe of Insight, Heaven's Halberd, Shiva's Guard, Necronomicon, Force Staff, Scythe of Vyse, Aghanim's Scepter.
Situational Items: Ring of Basilius, Vanguard, Crimson Guard, Blade Mail, Sentry Wards, Dust of Appearance, Gem of True Sight, Smoke of Deceit, Boots of Travel, Linken's Sphere.