Doom is an incredibly versatile melee strength hero, capable of fulfilling any number of roles from utility hero to hard carry tp jungler.
He is well known for his farming capability and his ultimate, Doom, which more often than not will take one enemy hero out of the fight, making teamfights 5 on 4 affairs.
Skills
You can prioritise any of Doom's skills, depending on what you want to get out of him.
Skill Devour first. Max it first if you want to maximise your GPM early on.
Skill Scorched Earth second if Jungling. Ptrioritise if you want AoE healing and damage over time.
Get an early point in LVL? Death if only for the mini-stun. Prioritise if you want burst magic damage on a low cooldown.
Always get your ultimate, Doom (levels 6, 11 and 16), and use to get early kills and to prevent skill-dependent enemies from getting off their combos in teamfights.
Items
Starting Items: A set of Tangos, a Stout Shield, a Ring of Protection, and an Iron Branch.
Utility Doom: Arcane Boots, Mekanism.
Ganking Doom: Phase Boots, Shadow Blade.
Jungle Doom: Hand of Midas, Phase Boots.
Luxury Items: Vladmir's Offering, Blink Dagger, Assault Cuirass, Crimson Guard, Heaven's Halberd, Necronomicon, Shiva's Guard, Black King Bar, Heart of Tarrasque, Drum's of Endurance, Aghanim's Scepter.