So, the text on Riposte reads:
"Play after a monster has attacked you with a melee attack."
whereas the text on Backstab reads:
"Play this card after the overlord has declared a monster's melee attack against you, but before he has rolled the dice for the attack."
and the text on Transparent reads:
"Play this card after the overlord has declared a monster's attack against you, but before he has rolled the dice for the attack."
In the latter two cases, the timing is clear, while Riposte seems to be either a) stuck with the playtest/less-clarified language but functions like Backstab and Transparent; or, b) intended to function AFTER a monster has COMPLETED an attack against you.
First game with Tomb of Ice tomorrow, so I'm not expecting a ruling before then, but I'd be interested to hear others' interpretation of the timing of Riposte.