1 : deadly or pernicious in influence baleful effects
2 : foreboding or threatening evil
1 The village could not survive the baleful effects of a poisoned well.
2 Her baleful look caused most people to leave the streets and shutter windows.
So, one of the more famous spells in fantasy rpgs is baleful polymorph. Why, because it’s funny and typically advantageous to turn the big bad guy into a tiny bunny or turtle in the middle of combat. Paizo added this word to the list so you could understand the spell’s nature though. Deadly or pernicious polymorph – think about that. It’s a spell that turns you into a small helpless creature that your opponent can now stab with a dagger, or a bbq spit. It’s not a just a spell used to stop the big threat, its a spell used to prepare your opponent for death.
Also, remember to use it in ways other than describing that spell, so when you use it on your party, they’ll get a feel for just how bad things just got.