The +10 is an additional modifier.

Dice Roll + Skill Bonus + 10

Late to this, but adding comments

Not a cantrip spell (sure only 2 ki points) but also how were you getting a +23 to stealth?!?

Additionally, you’re not invisible, just hard to spot, but a PC that rolled a 2 is still going to alert some guards whilst the rest of the party would not

I as a dm often have to tell players it is not  invisibility . The party has to make a dc test to use stealth  … a dc check in the forest will be very easy … say 10. A stone torch lit hall … with guards. 25 

Well with negatives 

Am I the only one that for years has been calling this "Pass Without *a* Trace?" 

Is the first line flavour or mechanics?  i.e. a Player casts this in a decently well lit area: would it not alert sentries seeing a 30 foot ball of shadow moving along the ramparts?

Cool spell and all, but with this and Summon Minor Elementals every stealth build is useless. Assuming with summon minor elementals you get a average of 2 chwingas, that alone is a +20 to stealth. If your DM runs a rule letting you pick your summons, thats a +80 to stealth.

Banger spell

I like how if you have a Stealth check of higher than +10, you can get a lowest possible stealth of more than 20. 

Let's say that I have 3 druids... Can the ability stack?