Just after the third and final session was finished.
The tattoo is based on the following bit from the book Serpent Mage (Vol. 4 of The Death Gate Cycle):
The dragon struck, pinioned the snake’s body. Claws sank deep into the scaled flesh.
The serpent writhed and twisted in fury. Its head coiled around, it snapped at the dragon, but the dragon was careful to keep just out of reach of the venomous jaws. […] The dragon, great wings straining, lifted the king snake from the ground, soared into the air. The serpent dangled from its claws.
The king snake fought, lashing with its tail, striking out again and again with its head.