Silver rat ring

What is it?

It is a magical piece of jewellery, an artifact. It's description reads:

This is a thick silver band, embossed with a circle of three rats biting on each other's tails.  Each rat has two tiny red rubies for eyes.
<number> of its eyes glow a dull red.
It emits a slight octarine glow.
It is in excellent condition.

It appraises as:

It is made of silver and appears to be a piece of jewellery.  It is in excellent condition.

What does it do?

You can wear the ring, and if you twist it and have sufficient skills (ma.it.wo.ring?), you will summon a rat that will protect you. This rat is a lot tougher than normal rats, though.

The number of charges seem to be equal to the number of glowing eyes on the ring.

How do I get one?

In the Rat Farm you will sometimes encounter a rat king. Once the rat king is defeated, the rats that scatter leave behind a silver rat ring. The ring always has the same amount of glowing eyes as the number of tails that the rat king had. This seems to always be between four and six.