Fanaelialae
Legend
The shadows have 19 hp, +4 to hit and need to take initiative, we talking about of a BBEG for lvl 15+ players, there is a solid chance to they die before get one hit. Great if BBEG have AoE. Even with two hits, is a lvl 8 spell to -5 BBEG str? Normally if the this BBEG is a Str dependent, he'll have a AC of 20+, if not, they only caution is kill the shadows before reach 0 str.
Actually, we are talking about a BBEG for level 11+ players. And I was assuming a 20 AC (that's a 25% hit rate for the shadows which, for 6 shadows, equates to 3.75 Strength drain per round).
Also (afb) don't shadows resist non magical weapons? Not all boss monsters have magical attacks, and if they don't 19 hp with resistance will probably soak two shots from the boss.
If the shadows get surprise and win initiative, they could easily do around 8 Strength damage before the BBEG can even act. If the bad guy doesn't have AOE (say he gets 3 attacks per round) then they can deal an additional 1.875 Strength drain for almost 10 points less Strength. -5 to all of a boss monster's attacks and damage? Yes please!
Now certainly, it won't always work out in such a solid fashion. Maybe the dragon spots them and roasts them in the opening round. Now you can rush the room and not worry about the breath weapon.
It's not unbeatable, but it's certainly solid, and I would expect it to trivialize fights now and again. The boss HAS to get the shadows down or else they will destroy him (unless he can stay out of their reach or has an AC that very few monsters possess). That means the party has at least a round to get into position without worrying about the boss. Easily worth a high level spell slot (otherwise you're probably wasting a slot on using up a legendary resistance use).