Lines are quoted from the official notes; buff/nerf tags are auto-classified from the numbers and can be wrong. Covers the 14 most recent Dota 2 patches PatchMeter has fetched.
▼nerfDamage gain per level decreased from +2.8 to +2.1
▼nerfDamage at level 30 decreased by 27 (from 149–153 to 122–126)
▲buffBase Attack Time worsened from 1.7s to 1.8s
▲buffBase Attack Speed increased from 90 to 110
✕removedRemoved Forsaken and Twist the Knife facets
✕removedRemoved Spectral innate ability
⇄adjustDesolateNow an innate ability. Passive, improves with Spectre's level
▼nerfDesolateDamage rescaled from 25/40/55/70 to 25 + 2 every time Spectre levels up
▲buffSpectral DaggerMana Cost decreased from 110/120/130/140 to 100/110/120/130
⇄adjustSpectral DaggerSpectre's illusions now also benefit from the Shadow Path
⇄adjustShadow StepReworked into a basic ability
▲buffDispersionMin Radius increased from 300 to 350
✕removedRealityCan no longer be cast to no effect
⇄adjustRealityNow has a distinct sound between casts on Shadow Step and Haunt illusions
▲buffRealityMana Cost decreased from 40 to 25
⇄adjustRealityNow has a 0.2s travel time to reach the target. The illusion and Spectre are invulnerable during this time
✕removedRealityNow disabled by roots
⇄adjustRealityNow always destroys the target illusion
⇄adjustHauntNow Spectre's ultimate ability
⇄adjustHauntMana Cost rescaled from 150 to 125/150/175
⇄adjustHauntDuration rescaled from 6s to 5/6/7s
⇄adjustHauntCooldown rescaled from 160s to 180/160/140s
▼nerfHauntHaunt Outgoing Damage decreased from 80% to 30/55/80%
⇄adjustHauntEach illusion now spawns from the direction closest to Spectre
⇄adjustHauntNow upgraded with Aghanim's Scepter
⇄adjustHauntReduces Haunt cooldown by 20s. When Reality is used on Haunt illusion for the first time per Haunt cast, enemies are Feared away from Spectre for 1.5s with 50% reduced move speed in a 400 AoE around the targeted illusion
▲buffTalentsLevel 20 Talent Spectral Dagger Slow/Bonus increased from +12% to +15%
▼nerfTalentsLevel 25 Talent All Spectre Illusion Damage decreased from +25% to +20%