no idea about stats but i like the concept, It is a little like gimmicky but that is not bad
Passive - Possession:
When Spectre would kill a monster (or pet or minion, though I’ll use the term monster for the remainder of this spell), the monster is instead put into stasis for 3 seconds (effectively stunned, but its still technically dead). If Spectre attacks a monster that is affected by Possession, he Possesses that monster, taking its body for himself (this counts as killing it). His stats become the stats of the possessed monster (including special abilities like the Wraith’s lifesteal) except for mana and health, with 25% of their max health instead of their full health, and retaining Spectre's Mana. Spectre’s items still apply their stat boosts, passive, and active effects. If Spectre does not Possess the monster, it dies after the stasis ends. When a monster Spectre is possessing dies, he appears where the possessed monster was with 75% of the health he had when he possessed the monster.
This does NOT work on Baron Nashor, Dragon, or other similar “boss” monsters, but it does work on Buff Monsters like Golem and Lizard.
does it work on annie bear cause that might be op, I would add a cooldown to the whole passive, but also how do you posses, this seems more like an ability, also maybe add a max time of possession that scales with level
Q - Ecto-Blast:
By pushing his ectoplasmic magic into his hand, Spectre fires an energy blast at target enemy dealing 85/130/175/220/265 (+10% bonus Health) Magic Damage. If used while possessing a body, this costs an additional 2.5% of that body’s current health, and deals an equal amount of True Damage to the enemy hit.
Cooldown: 6 seconds
"10% bonus health" is that spectre's or the target. good i might add scaling cd
W - Spectral Aura:
Passive - Gathering his ghostly energy, Spectre forms an aura of power around himself. His auto-attacks deal 20/30/40/50/60 (+5% of the max health of a Possessed body not counting items, +2.5% of your bonus health) bonus Magic Damage. This is disabled while the skill is on cooldown.
Active - Spectre charges up the power of his aura, then releases it in an explosion, knocking all enemies within 200 units back 400 units and dealing 125/155/185/215/245 (+5% of the max health of a Possessed body, +2.5% of your bonus health) Magic Damage to all enemies hit.
Cooldown: 10 seconds
i don't like this ability, i would move the passive here get rid of this and find a new passive
E - Invisibility:
Using his natural spectral powers, Spectre turns invisible after 1.25 seconds (delayed upto 3 seconds by damage), taking any body he might be possessing with him, for 4/6/8/10/12 seconds. If this stealth ends while possessing a body, the first auto-attack deals 120/140/160/180/200% damage instead (this is treated as bonus damage, so the extra damage does not crit). If the stealth ends while not possessing a body, Spectre gains 20% bonus Movement Speed for 3/3.5/4/4.5/5 seconds.
Cooldown: 20 seconds (as people have commented on this CD being short enough that CD can "allow for permastealth", I'd like to point out CD doesn't start on stealth until it ends)
seems to fit the character well, not much to say
R - Intangibility:
Taking advantage of his ghostly nature, Spectre can turn himself (and any body he is possessing) Intangible, granting him immunity to damage, ignoring unit collision, and allowing him to move through terrain for 2/3/4 seconds. If he is still in terrain when this effect ends, he is immediately moved to the nearest empty space. Spectre can only move through 1 terrain per use of this spell (1 terrain meaning after he enters a terrain, once he exits that terrain, he cannot enter another terrain for the rest of the duration).
Gathering his power after letting himself become intangible for a moment, Spectre’s next attack (spell or auto-attack) within 4 seconds deals 50/100/150 bonus Magic Damage. If he is possessing a body when Intangibility is activated, the body explodes, dealing it's remaining health as Magic Damage to all enemies within 300 units.
Cooldown: 150 seconds
i like the idea, again not much to say but i would make the next attack thing stun and deal less damage
Overall: good job, a few problems, one you didn't scale cd or mana on most of them if thats what u want leave it but..., and no scaling of AD or AP in the ablilies, every champion needs that
if you need clarification just ask and here are my champs
here are some passive that might work if you moved the passive to an ability
-after not attacking for 10 seconds his next ability will taunt for .5 of a second
-every basic attack gains a shield of 5% of the damage for later, when under 30% health Spectre gets this shield