This is the concern that prompted Glenn's suggestion. The idea was
that these artifacts could be used a finite number of times, after
which they would melt away to be regenerated somewhere else.
I was thinking that you'd USE the artifact to get the beasties to
pop up. They'd hang around for a game year or so, then melt away.
The artifact would be capable of doing this three times, after which
it would vanish.
Your idea is actually considerably simpler. Just don't regenerate
the beasties when they die, and when the last one dies, the artifact
melts away and is regenerated elsewhere.
-- Rich Skrenta <email@example.com>