This is an excellent starting point - I already have some thoughts to flesh this out somewhat.
1. The ring dates far back, and was used to lend fact to a legend that 'even the very beasts flocked to the King's standard..'
2. The ring does not provide simple control as described, but causes the ring-wearer to become the pack leader. This is two-edged, for the pack leader can lead the pack, and they will follow - perhaps accepting some verbal orders through the power of the ring. Second, should the new leader show signs of weakness, up-and-comers (or the old leader) may attempt to regain control. I'd personally make it as follows:
Upon donning the ring, the bearer assumes a greater and greater resembles to wolves as the pack is summoned to the bearer. The wolves will travel at their best speed to reach the wearer, intruding into civilized regions if need be. If that pack is destroyed, then another and another will be so drawn until at least one wolf survives to greet the bearer.
The pack, upon arrival, may accept the new bearer automatically if 'strong', otherwise the current pack leader will challenge the wearer by attacking him. The other wolves will not intervene unless attacked. Should the bearer fail, the pack will tear him to shreds.
Once the bearer is the pack leader, he can shift between wolf-man and pure wolf forms, the latter needed to take advantage of the wolves faster movement. Human form cannot be resumed without removing the ring.
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Removing the ring could either be hazardous or safe at the GM's discretion.
I think this could even be used as the start point of a campaign, if the demon on remains long enough to scare the bejesus out of the PC's.
Other aftereffects of this occurrence could then make things move very quickly. Perhaps the PCs have seen far too much and there are those who would seek to silence them.