Psychopomps are some of my favorite outsiders, and I've been trying to get a memitim eidolon made for PFS for a while. Problem is, unchained eidolons don't get any spell-like abilities and have no evolutions that affect manufactured weapons. So far, I have:
1 - Take Extra Evolution and get Simple Weapon Proficiency, Weapon Focus (scythe) for the feat, immune to death effects, disease, and poison, and has darkvision 60' [2/2 EP]
2 - Natural Armor Increase, Evasion [3/3 EP]
3 - Power Attack [3/4 EP]
4 - cold resist 10, electricity resist 10 [3/5 EP]
5 - Take Extra Evolution, Flight, +1 Str [5/7 EP]
6 - Cleave, Devotion [5/8 EP]
7 - [5/9 EP]
8 - Spirit Touch [5/11 EP]
9 - Spell Resist, Great Cleave, second natural attack [9/12 EP]
10 - +1 Str [9/13 EP]
11 - Take Extra Evolution, Frightful Presence, Improved Crit (scythe0 [12/14 EP]
12 - DR 5/adamantine, Spirit Sense [12/15 EP]
So do I just chuck the Ability Increase and Natural Armor evolutions on for the remainder of the evolution points? Seems pretty lackluster.
I almost think forgetting the entire psychopomp theme and spamming natural attacks and pounce would be more effective. It seems like actually building an unchained eidolon to fit an outsider is impossible.
Also, Extra Evolution can be taken at 1st, 5th, 10th, 15th, and 20th levels, but Summoners don't get a feat at 20 <shakes head>
Summoners get a feat at 21, and there are rules for post 20 leveling. Though most campaigns don't even make it to 20, and PFS typically ends in the 12 range.
And, yeah, it's very difficult to get the exact flavor you might want out of the Unchained Summoner, especially with the limited outsider types (and the general inability for you to have an outsider that doesn't conform).
However, your concept here looks very interesting, and I like where it's going. Taking Ability Score increases and natural armor might sound very boring, but something that is just stable benefits, even if boring, can be used to supplement all the cool abilities.
And I just realised that scythes aren't simple weapons, don't know why I thought they were. Guess we'll skip Great Cleave and push the other feats back a level to take Martial Weapon Proficiency.