Spectre is a melee agility hard carry. Whilst she is a late-game monster, capable of potentially destroying whole teams, Spectre is a very greedy carry who requires immense amounts of farm in order to fulfil her late game potential. For this reason she is a relatively unpopular pick outside of competitive Dota.
Skills
Spectral Dagger is always skilled first. It can be maxed either first or second, depending on personal preference.
Desolate is either maxed first or second, depending on personal preference.
Dispersion is always maxed last.
Haunt is always skilled (levels 6, 11 and 16), and used to join teamfights or secure kills on low enemies.
Items
Starting Items: Tangos, Healing Salves, Iron Branches, Quelling Blade or Stout Shield.
Early Game: Power Treads or Phase Boots, Magic Wand.
Core Options (Pick One): Radiance (greedy), Manta Style (early combat), Diffusal Blade (early haunt damage)
Luxury Items: Diffusal Blade, Manta Style, Butterfly, Heart of Tarrasque, Abyssal Blade, Daedalus.
Situational Items: Vanguard or Drums of Endurance (early tankability), Crimson Guard, Hand of Midas (early greed), Blade Mail (poor start), Black King Bar (mid-late game magic immunity), Boots of Travel (late game).