Hi, I haven't worked on TrinityCore for 3 months now, but I'll give you a definite answer. I worked on a creature deathsystem, on the technical side I decided to phase all the creatures instead of kill/delete them from the DB, to avoid abuse. So I messed with phasing and found that your solution is right but only works untill the next server restart because you're changing the phase of a creature in memory only, not through the Database.

void OnPlayerEnter(Player* player) OVERRIDE
{
if player->HasSpell(95000)
Creature* temp = instance->GetCreature(CreatureGUID);
temp -> SetPhaseMask(x01, false);
}
Like I said, this is only a temporary solution, if you want to permanently add a NPC to a phase then this little snippet of code is required.

killed->SetPhaseMask(8, true); //Put NPC in phase 8
WorldDatabase.PExecute("UPDATE creature SET phaseMask = 8 WHERE guid = %u;", killed->GetGUIDLow()); // Update the database to set the creature's phase mask.
I've tested this snippet and it works.

So, here is your slightly corrected code,
void OnPlayerEnter(Player* player)
{
if (player->HasSpell(95000))
Creature* temp = instance->GetCreature(CreatureGUIDLow);
temp -> SetPhaseMask(1, true);
WorldDatabase.PExecute("UPDATE creature SET phaseMask = 1 WHERE guid = %u;", killed->GetGUIDLow());
}
Again, the executing to database is if you want to make the NPC stay there permanently, just remove the SQL line if you want the NPC to be temporary.

I've done this before on Trinitycore 3.3.5a, I don't know if you're using Trinitycore 3.3.5a or not, so I'll just explain what I did.

I edited StableSlotPrices.dbc, and I figured out that editing the client side .DBC is not enough, you need to update your server-side .DBCs as well. Or else if you just edit the client side, it will only visually change the price but not actually change it server-side. So remember to update your server-side .DBCs

I'm going to try this out and tell you if I used/implemented everything right. I was bummed out when I got Milly's darker nights to find out torches don't actually produce light on their own. Now I have the power to change that, thanks.