I'm working on a homebrew GURPS 4e fantasy campaign (intended to be played online, though I don't think that matters for this question). One of my monster concepts has an ability that's easy to describe, but I'm having trouble figuring out how to write it up and calculate its point cost.
This is a plant-based monster with Basic Move 0. It detects motion within a set distance (twice the actual hex size of the monster), and casts seeds to that radius. The seeds burst on striking anything hard (the monster ensures there will be hard objects around), and when they burst, release a cloud of dust that, when inhaled, acts as a mild, slow-acting soporific. Any creature in the affected area must roll against HT each combat round or start suffering effects.
On a failed HT roll, the dust will immediately cause an Affliction (B39), beginning with the Tipsy effect (-1 to DX and IQ, -2 to self control rolls except to resist cowardice), after one minute (and another HT roll at -1) this will escalate to Drunk (-2 to DX and IQ and -4 to self control rolls except to resist Cowardice), after two minutes (and a third HT roll, now at -2) to Euphoria (-3 to all DX, IQ, skill, and self control rolls), and after five minutes (and a final HT roll at -3), to Paralysis -- at which time cessation of the victim's movement causes the attack to cease, allowing the victim to recover over a time frame determined by the level of failure on their initial HT roll.
Obviously, this is a Respiratory Agent (B108), with Area Effect (B102) (4 yards or 8, depending on the size of the monster). What makes it hard to calculate is the reversed order of the Affliction -- an Irritant effect like Tipsy, Drunk, or Euphoric would normally occur after the Incapacitation (Paralysis) wears off. The gradual onset looks like Cumulative (since the attack continues as long as one or more victims are moving within the detection range), but also like Onset (because it takes time to escalate).
How would I build this attack (ideally using GCS, since I'm also trying to learn how to build Powers in GCS)?
Here's my take on writing it up, but this point total seems very high for something that would probably only trap animals and those who are completely unaware that the wilderness is a hazardous place.
Affliction 4: (10/level)
Area Effect (2^level yard radius) +50%
Respiratory Agent: +50%
Limited Use: 2/day: -30%
= 312 points according to GCS.