From Discworld MUD Wiki
It is possible to fail invoke, you'll get this message
Despite your best efforts, you can't seem to invoke the power of a gold-veined keris and you feel you have poured rather too much of your vitality into the attempt.
You'll take about 420 damage when you fail
I guess there's a skill test, and I think it is ma.it.talisman? (I have 100/204)
Research needed on this