Illusion (phantasm) is a school (subschool), not a descriptor. Can Su abilities have that?
It can have it if we want it to have it!
I'm ok with Cha 15. But these are going to be really low CR to have such a nasty effect. I mean, even 3d6 has a decent chance of killing a character at the likely CR. I'd like it to have a fairly low chance of succeeding (much like the cockatrice's petrification).
Well the original scenario is for 4th to 6th level characters, so ideally we want something that's a moderate threat to a single character of that level. (The way the adventure was set up it was one PC vs. one Nightmare Rat, and it didn't matter much if the Rat killed them since it was "only a dream").
So, assuming it's the lowest level (4th), the adventurer is likely to have enough hit points to survive the damage (even a wizard with an average Con score has 10 hit points and a 50-50 chance). Their good save is probably about +5 or +6 (base of +4 for levels, with at least a few extra points from abilities or resistance items) but their bad save might only be +1.
If we use plain HD + Charisma for the Rat's nightmare DC and assume it has Cha 14-15 then the nightmare bite will have DC 12.
What's the weakest opponent the Rat is likely to face?
I'm thinking Rogue, since they have poor Will and Fort save and mediocre hit points.
Let's see, I'll assume a 4th level Rogue with saves of Will +2 and Fort +2 and 14 hit points.
If this rogue gets bitten there's a 55% chance they'll save completely, a 24.75% (0.45*0.55) chance they'll fail the first save and take 3d6 damage and a 20.25% (0.45*0.45) chance they'll fail both saves.
In the latter case they die. In the 3d6 damage case they'll drop to the Dying condition if the damage roll is 14+ (enough to put their initial 14 hit points into the negatives, allowing for the 1 point of damage the bite itself does). The chance of that happening is 35 in 216, or about 16.2%. Multiply that by the 24.75% odds of them failing the first save but not the second and they've got an approximately 4% chance of being knocked into "Dying" by the first bite. (0.162*0.2475)
So, if we add the odds of "Dead" (20.25%) and "Dying" (4%) together, the weakest likely opponent has around a 1 in 4 chance (24.25%) of being killed or mortally wounded out by the first bite that hits.
Contrariwise, a strong 4th level opponent will have 30+ hit points so the damage doesn't pose an immediate threat, meaning they only go down if they fail both saves. Such a foe's saves are likely both good. Let's say +6, for a 75% chance of success. That works out to a 6.25% chance they'll fall to the first bite and an 18.75% they'd take the additional venom damage. On average it'd take roughly a dozen bite attacks to kill them from "single save failure" poison hits - 11 hits with its bite attack at 18.75% chance. With that many hits there's a 49% chance they'd roll a double save failure and get the "dies instantly" result.
More importantly, it's
highly unlikely the Nightmare Rat would survive long enough in melee combat to get eleven hits in. It'd be lucky to hit twice before its opponent bashes through its 8 hit points.
Also, remember they don't have any Reach so most opponents will get an opportunity attack in unless the Rat surprises them.
Are those the kinds of odds you're after? I'd prefer something a little nastier. Maybe give it a +2 racial bonus or Ability Focus (phantasmal killer poison)?
Then again, I'd probably be using my Cleon Special Homebrew if I ever wanted to run a Nightmare Rat, so the issue is a bit academic.