2 Level 3 EARTH monsters
Once per turn: You can detach 1 Xyz Material from this card to target 1 Set card in your opponent's Spell & Trap Card Zone; it cannot be activated while this card is face-up on the field. If this card destroys an opponent's monster by battle while it has Xyz Material: You can target 1 EARTH monster in your Graveyard; Special Summon that target in face-up Defense Position.
So, this card is similar to Fairy King
Soul of Silvermountain has a big ally in Wind-Ups. With Wind-Up Rat and Wind-Up Rabbit, 2 of the key Wind-Up monsters, being Level 3 EARTH monsters, Silvermountain is an easy addition. Rat's effect essentially turns itself into a Silvermountain, since you lose no card advantage. Since Wind-Ups are so big on swarming, Silvermountain is able to eliminate possible threats before going in for a big assault.
A deck that can really make use of Soul is X-Sabers. Being composed of several key EARTH Level 3s, X-Saber decks can churn out this Xyz easily. Better yet, almost all of their Level 3s make Xyz Summoning easier. XX-Saber Fulhemknight can not only protect itself for a turn, but it can Special Summon another monster from the Grave. XX-Saber Emmersblade and XX-Saber Darksoul are both floaters, meaning they'll keep grabbing more materials. Since X-Sabers resemble Wind-Ups in their strategy of swarming for the kill, Silvermountain will similarly be used in such a deck. Of course, if it manages to take blood from your opponent, he'll summon out more powerful Sabers from the Graveyard, even XX-Saber Gottoms!
Fellow REDU cards, the Madolches, will also work nicely with Soul. Their Madolche Mewfeuille will be able to Special Summon another copy of itself or Madolche Baaple from your hand, meaning insta-Xyz. A nasty move to do is to summon Baaple and switch the position of one of your opponent's low DEF monster with its effect, as well as Mewfeuille, and then Xyz into Silvermountain. From there, you can just attack and use Silver's effect to add one of your EARTHs to the field. (hinthint: Give your opponent a taste of the Bitch Puddin'. BLAYUM!)
There are also several other rogue ways of summoning this card, ranging from Gilasaurus, Marauding Captain, and even in Nordics. It's EARTH Attribute, so really it isn't THAT hard to get out. It has a myriad of uses.
Silvermountain does have one fatal flaw though, and that is that it can be chained to. That really stings its usability in some situations, since all you could end up doing is forcing the activation of Compulsory Evacuation Device or Mystical Space Typhoon.
Usability: 7/10, a decent card, but it's hurt by the current format where many things are chainable.
Art: 8/10, nice mountains