Not entirely in the scope of this thread, but an interesting idea nonetheless. I wouldn't want that personally, unless you know you are gonna be bothering a mighty powerful lich before you do it. and still i'd be disappointed i just dont... you know...fight it.
Hmm... perhaps you hear the Lich whispering warnings about trespassing upon unholy ground that become more dire as you enter the heart of its lair?
The reward could be disposable or a spell to summon the Lich once a day, but he will be frenzied so kind of an 'oh crap!' spell.
EDIT: The ritual encounter should be a really tough fight since you have to perform the ritual under pressure. I am thinking of waves of undead. 1 super tough minion per wave. It would be like a gauntlet I guess, maybe.
EDIT2: Since you have to run around his lair for the ritual perhaps there would be 1 baddy after each completed event. The lair could be sealed once you initiate the ritual so that you have to do the whole thing at one time.