What is a Vecna Offering?

Double click the skulls token. It will vanish and a bag will appear in your backpack with a random amount of Vecna Coins and minting items

You can then either take them to Britain Bank, and roll Vecna's Corruptible Dice for a chance at [event task cooldown or lowering daily task requirements (provided you haven't already done one that day)

Or you can go to Ilshenar Dungeons>Vecna's Tomb.
In the first room, there is an Altar. Walk up to it and say "Vecna Lives" for a random
reward

Vecna may reward you or curse you based on how worthy you are!