Mostly all good.
Gnome Cunning, Stone Camouflage, Darkvision out to 120' all works.
For me, there are two issues...and they are slightly related.
1. Sunlight Sensitivity. That's a big fat yes! They are Underdark creatures. They have 120' Darkvision. There is nothing in the MM to indicate these guys are running around on the surface ever. So, for a PC race, yes, you have Sunlight Sensitivity.
2. The Innate magic. See, the Drow have an at-will cantrip. A 1st level effect at 3rd. A second level effect at 5th. Trouble is, the svirf's have no cantrips, 1 1st level effect,
2 2nd level effects, and a
3rd level effect.
Here's what I'd do...the 3rd level effect is Nondetection...that is an at will of the race. For a PC adventurer guy, and in keeping with previous editions lore, that gets moved off the spell list to be a set racial trait:
Inscrutable: The PC can not be targeted by divination magic or perceived through magical scrying sensors.
There, Nondetection is off the table/out of the way while still contributing something special to the PC in a way keeping with the lore.
Now, we're still left with no cantrip, the 1st level effect (going into 3rd level, obviously), and 2 2nd level effects. So, is blur or blindness/deafness the more "iconic" svirf power and/or can those be achieved in some other way? Is there a cantrip power, that giving it to the pC would not be overpowered?
After some research. It seems neither can be simulated by cantrip effects, without significant alteration of the svirf history/lore. Which means the spells and their effects will have to stay as is and have to be sorted as best we can. So I would propose:
At will:
Blade Ward cantrip. (Goes with the whole race of warriors/taking down enemies twice their size. Resistance to slashing, bludgeoning and piercing damage in fights against drow, kuo-toa and duergar seems like it might come in handy. As well as dovetailing nicely with the flavor/fluff of them being extra dense/heavy/hard and tough.
At 3rd level:
Disguise Self 1/day. (which, ironically seems like the most potent spell effect)
At 5th level:
Blur 1/day. (in keeping with the svirf knack for being difficult to find)
I would, here, deem that by virtue of making the 3rd level spell effect up all of the time, and the keeping-with-flavor cantrip, that's all the PC svirf gets. No, it's not the exact same as the NPC race. PCs aren't supposed to be.
Optionally, if you feel you must, at 7th level: Blindness/Deafness 1/day.
SO, the final [that is, "Steeldragons' variant" if Steeldragons were going to allow svirf PCs -which he never would unless playing a whole Underdark campaign/game] Svirfneblin PC race looks something like this:
Ability Score Increase, Age, Alignment, Size: as Gnome
Speed: slowed to 20', as per MM.
Darkvision: out to 120', as per MM & drow.
Gnome Cunning: as Gnome.
Languages: Common, Gnomish.
Sub-race: Deep Gnome.
Deep Gnome, Svirfneblin
Ability Score Increase: Your Str. score is increased by 1.
Bonus Languages: Terran, Undercommon
Stone Camouflage: as per MM
Inscrutable: as above.
Sunlight Sensitivity: as Drow.
Innate Magic: At will:
Blade Ward. @3rd:
Disguise Self 1/day. @5th:
Blur 1/day.
Even with the sunlight sensitivity and removing the blindness magic, I would be nervous about this being OP'd in relation to other PC races. But, given the magics of Drow and Tieflings (and presumably other "optional" races I don't really remember from the DMG preview, like Aasimar and 3-4e Eladrin )...perhaps not. [EDIT: upon some reflection, I REALLY dig the flavor of the Blade Ward a, like, the Martian Manhunter/Vision trick of becoming extra-dense to protect themselves. But all told, it still might be OP compared to other PC races...not necessarily Underdark races, but "normal" PC races, I mean./EDIT]
So...there's mine.
