poorquentyn:

Means:

“To be sure, I have much to thank your sister for. If not for her
accusation at the feast, it might well be you judging me instead of me
judging you.” The prince’s eyes were dark with amusement. “Who knows more of poison than the Red Viper of Dorne, after all?”

“Widow’s blood, this one is called, for the color. A cruel potion. It
shuts down a man’s bladder and bowels, until he drowns in his own
poisons.”

The King’s Hand was rotting visibly. His face had taken on a greenish
tinge, and his eyes were deeply sunken, two black pits. Fissures had
opened in his cheeks, and a foul white fluid was seeping through the
joints of his splendid gold-and-crimson armor to pool beneath his body.

Motive:

“I have hungered for a long time. Though not for food. Pray tell me, when will the justice be served?”  

“Your father,” said Prince Oberyn, “may not live forever.”

Something
about the way he said it made the hairs on the back of Tyrion’s neck
bristle. Suddenly he was mindful of Elia again, and all that Oberyn had
said as they crossed the field of ashes. He wants the head that spoke the words, not just the hand that swung the sword. “It is not wise to speak such treasons in the Red Keep, my prince. The little birds are listening.”

“Let them. Is it treason to say a man is mortal? Valar morghulis was how they said it in Valyria of old. All men must die. And the Doom came and proved it true.”

Opportunity:

“Where will I find my lord father?”

“In the solar with Lord Tyrell and Prince Oberyn.”

Mace Tyrell and the Red Viper breaking bread together? Strange and stranger.