Eidolon Parasite (Unchained Summoner archetype)
An eidolon parasite drains strength from their eidolon, reducing it to a shadow of its potential while becoming ever more monstrous with stolen power. The overwhelming majority of these eidolons will hate and fear the summoner that has parasitized them, though a handful might take a perverse pride in the achievement.
Alignment: An eidolon parasite with a good or neutral eidolon must be evil, and one who becomes non-evil loses his evolution pool and will become an unchained summoner with no archetype the next time he gains a level.
Parasite Pool: A eidolon parasite’s eidolon gains half as many base evolution points as a regular eidolon of its level (minimum 1), its Str/Dex bonuses are half of their listed values, and its Max attacks are reduced by 2. It does not gain the share spells or devotion abilities.
An eidolon parasite gains an evolution pool equal to the evolution pool of his eidolon, not counting increases from its subtype or the Extra Evolution feat. He may ignore requirements based on subtype but must otherwise meet the requirements for any evolution he gains, and for the purpose of qualifying for evolutions an eidolon parasite has the biped base form and begins with the limbs [legs] and limbs [arms] evolutions. He cannot select the ability increase evolution through this ability. The eidolon parasite can change his evolutions whenever he gains a level (or through the transmogrify spell). This ability alters Eidolon.
Stolen Flesh: At 1st level, the eidolon parasite can cast a spell with a target of “your eidolon” on himself as though he were his own eidolon and gains Toughness as a bonus feat. An eidolon parasite cannot sacrifice his own health to prevent damage to his eidolon. This ability alters Life Link and replaces Life Bond.
Begrudging Cooperation: At 3rd, 9th, and 15th levels the eidolon parasite and his eidolon gain a teamwork feat as a bonus feat. They must both select the same feat and must both meet the requirements for it. This ability replaces Summon Monster.
Otherworldy Might: At 4th level the eidolon parasite gains a +2 bonus to natural armor, +2 bonus to saving throws, and a +2 bonus to Strength and Dexterity. At 12th level these bonuses increase to +4. This ability replaces Shield Ally and Greater Shield Ally.
Ruthless Transposition: At 16th level the eidolon parasite can use Transposition as an immediate action when he is hit by an attack. If he does so his eidolon takes the damage in his place. This ability replaces Merge Forms.
Consume the Weak: At 20th level as a full round action the eidolon parasite can consume his eidolon, killing it to rejuvenate himself as though affected by a heal spell. When he does so he gains the benefits of its 1st, 4th, and 8th level base evolutions. He may benefit from these evolutions even if a creature of his base form would not normally qualify for them. These benefits last for 1 hour.
Obviously this is aiming at an upfront combat focused summoner archetype more akin to the synthesist, greatly weakening the eidolon but not sacrificing it entirely and losing so much in the action economy, but also not overriding the summoner's physical stats and giving them the full eidolon toolkit.