GURPSよろず

　You are, knowingly or not, a human / Deep One hybrid.
At some point in your life you will lose your humanity, become a Deep One and go to live beneath the sea.

　There's a definite "fishy" look to individuals with this disadvantage - skin may have a translucent, greenish cast, eyes may be slightly protuberant, fingers or toes may he webbed. Characters with the Innsmouth Look cannot start the game with better than Attractive Appearance.
As they mature, their appearance becomes more inhuman - skin becomes scaly, hair starts to fall out, hands and feet start to splay.
Characters lose one level of Appearance every 1d years (rolled by the GM).

　The change itself occurs when the character is 18+5d years old (the GM may, of course, select the time of the transformation rather than rolling it randomly).
The nascent Deep One need not have deteriorated all the way to Hideous Appearance - many half-breeds change when their appearance is still predominantly human.
The actual change from land-living human to aquatic Deep One is relatively quick - once the change begins, the victim will completely be a Deep One in 2d months.
The actual process of the change within this time period is left to the GM.

　Some half-breeds do not change all the way, but for the purposes of this disadvantage, such wretches are treated no differently from anybody else with the Innsmouth Look - they will still come to a time of change when they will become something other than human.
Some half-breeds never change at all - they do not have this disadvantage.

　It can be assumed that after the change, the character's personality also changes.
He will nor become a Deep One Ally of the PCs, much less an amphibious member of their fellowship. He will, at best, no longer care about humanity or his former friends.
If they attempt to keep him from joining his undersea relations, he will go to any lengths to escape.
In short, barring extreme acts of GM's fiat, once the change comes a character with the Innsmouth Look is permanently removed from the campaign.

　The Innsmouth Look can be recognized by those who know about the human/Deep One crossbreeds.
Roll against IQ - 5 or Physiology skill -3 (these defaults apply only to characters who know that the Deep Ones exist).
Characters who have dealt with crossbreeds before can recognize the condition with an IQ roll ("Gee, that guy looks a lot like the guy at the grocery store.").
Of course, recognizing the family resemblance doesn't give any information about what family they're dealing with . . .
The roll is modified by +2 for every level of Appearance that the subject has deteriorated below normal (i.e., an attempt to recognize a half-breed of Hideous Appearance is at +6), and by Vision modifiers and Alertness.