90. Picard accepts the role as Arbiter Of Succession in a struggle for two warring Klingon factions to gain control of the Klingon Empire, shortly before the one extending the invitation to him dies in a chair.
From Quiz Name That Episode #15
Answer:
Reunion
At the request of the Klingon ambassador, K'Ehleyr, Picard meets with the leader of the Klingon High Council onboard the Klingon cruiser who is dying from prolonged poisoning. Before he takes his last drink of wine and dies in his chair, he tells Picard that he's chosen him to be the arbiter between the two factions vying for control of the empire (led by Gowron and Duras, respectively), due to the captain's positive reputation for strong diplomacy. Initially, Picard declines, stating that he should've been informed before the decision was made. But after further consideration, weighing the potential involvement of the Federation in a possible Klingon civil war, Picard eventually accepts, then later proceeds with being in charge of the Rite Of Succession ceremony back onboard the Enterprise. By the time the fourth-season cliffhanger "Redemption, Part 1" aired, Picard was already in his role as arbiter for the Klingon High Council, and a determined Gowron reminds Picard of that when informing the captain that the Duras Sisters are plotting leadership of the High Council. In "Sins Of The Father", Picard does accept the role extended by Worf to be a Cha'dich and stand at Worf's side before the High Council, but the Arbiter Of Succession role is never presented to the captain. Then in "A Matter Of Honor", there is no Klingon role that is extended nor bestowed upon him at all.