So, it's an 8th level spell, which for 1 minute/level renders the critter immune to detection by anything except true seeing.
Clearly it shouldn't be thrown at PCs who couldn't have access to true seeing, which is a 5th, 6th and 7th level spell... let's call it 7th level just in case the party has a Sorcerer + Druid instead of Cleric + Wizard, or in case the Cleric is a cohort.
So. The critter should be stealth-oriented, deal Sneak Attack or Sudden Strike damage, and be around CR 15, or CR 13 if it attacks in pairs. Clearly this spell-like ability just by itself won't justify the CR... but that's the range of the package you should be looking at, IMHO.
Cheers, -- N