This is an attempt to make the fossegrim into a playable race while attempting to capture the monster's mechanics as closely as possible.
The class is definitely on the high-power side and since it's worth exactly 20 RP, DMs are easily able to justify either giving it a +1 level adjustment until level 6, or keeping it as is.
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Type: Fey (2 RP).
Subtype: Aquatic (0 RP, obtained upon meeting the qualifiers from Swim + Amphibious).
Size: Medium (0 RP).
Speed: Normal (0 RP).
Senses: Low-light vision (from Fey type) (0 RP).
Ability Scores: Advanced (4 RP). +4 to Cha, -2 to Wis, +2 to Str, Dex and Con.
Languages: Standard (0 RP). Common and Sylvan.
Racial Traits:
DEFENCE
Fey Damage Resistance (3 RP): DR 5/Cold Iron.
MAGICAL
Seducer (2 RP): Adds +1 to the saving throw DCs for their spells and spell-like abilities of the enchantment school. Members of this race with a Charisma score of 15 or higher gain the following spell-like ability (the caster level is equal to the user's character level): 1/day—charm person.
Greater Spell-Like Ability (3 RP): May use Water Walk 1/day, with character level as CL.
Fascination (3 RP): 1/day—a member of this race can create a 20-foot-radius burst that causes humanoids within the aura's range to become fascinated with the user (as the bard's fascinate bardic performance). Affected humanoids may resist this effect by making a successful Will saving throw (DC 10 + 1/2 the user's character level + the user's Charisma modifier).
MOVEMENT
Swim (2 RP): Swim speed of 30 feet and gain the +8 racial bonus on Swim checks that a swim speed normally grants.
OTHER
Amphibious (2 RP): Breathes air and water equally.
WEAKNESSES
Weakness to Flattery (-1 RP): Fossegrim take a -4 racial penalty to Sense Motive, Perception, Bluff, Intimidate and Initiative checks for 1d4+1 rounds when a creature succeeds at a DC 20 Diplomacy check to flatter them. Creatures cannot make multiple Diplomacy checks to stack these penalties. Further successful Diplomacy checks to flatter merely reset the duration.
Alternative Racial Traits:
The following traits reference the fossegrim Bestiary entry.
Will to Kill: Fossegrim may gain the Drowning Touch ability 3/day along with Cold Iron Vulnerability (take double damage from cold iron weapons, attacks or effects). This replaces Seducer.
What Glitters In The Depths: Fossegrim may gain the Treasure Form ability 1/day. This replaces Fascination.
Out of Sight, Out of Mind: Fossegrim may become transparent underwater 1/day. Its physical form still exists and it is not incorporeal—only its appearance changes. This racial trait works like invisibility, except the effect only lasts 1 round per level (maximum 5 rounds) and may only be used while completely submerged in water. This replaces the Advanced Ability Scores. Fossegrim with this racial trait only gain a +2 to Cha and Dex, and still have a -2 to Wis.
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Feedback appreciated!
What is this race from?
I actually added a link in the OP, but the colour is not very visible.
It's This Bestiary Creature turned into a playable race using These Rules.
The Alternative Racial Traits are purely ad-hoc and therefore basically eyeballed in terms of balance. It's entirely possible that Vulnerability to Cold Iron doesn't make up for having a save or die 3/day at 1st level! I consider that since it offers a bunch of saves before actual death, and the saves scale of Con instead of Cha, it's roughly acceptable.