All right, this story from an earlier answer of mine is outside your time frame by a decade or so, but I think it has everything you're looking for in abundance:

You know how there are some stories that are just waiting for their soapy, bloody, sexy HBO miniseries?

Byzantine emperor Michael VIII (r. 1259-1282) was, among other little things like reestablishing the Byzantine Empire, pretty much the champion of making alliances. This included sending two of his illegitimate daughters north and east to marry Mongol rules: Maria married Abaqa Khan of the Ilkhanate, and Euphrosyne married one-eyed Nogai (Nogaj) Khan of the Golden Horde.

What this did not include was dealing with the troublesome Bulgars/Bulgarian Empire, an off-and-on subject people of the Byzantines whom Michael strongly wished to remain "on." With a revolt looming in the late 1270s, Michael instead married his daughter Irene to an exiled Bulgarian, Ivan Asen III, and dispatched him to Bulgaria at the front of an army to take the country back by force.

Ambitiously cold-blooded Hungarian-Bulgarian noble George Terter seized the opportunity of a new tsar: he summarily divorced his wife Maria to marry Ivan Asen's sister (also named Maria). To prove his loyalty, he packed his ex-wife and son Theodor Svetoslav off to Nicaea (i.e. in the Byzantine Empire but at a safe remove from power) as hostages.

Ivan Asen proved to be no better this time around, and another revolt in 1280 sent him fleeing back to Constantinople once again. George was pleased: he seized the Bulgarian throne for himself. With some leverage over Michael, now--I promise I won't revolt, too--he brought the first Maria and son Theodor back to Bulgaria, even making Theodor co-emperor.

Well, remember what I said about cold-blooded? The Mongols got pesky again in the middle of the decade. George's response? He sent Theodor and sister Helena off to the Golden Horde, where Helena married Chaka Khan and Theodor seethed as a hostage yet again. In an effort to seize back a modicum of the power he had once wielded, he finagled a marriage alliance for himself with the daughter of Nogaj Khan and (Emperor Michael's daughter) Euphrosyne, also named Euphrosyne because why not.

So now we're at three and a half Christian-Mongol marriage alliances, and the real fun can start.

George Terter proved to be only marginally better at wrangling Bulgaria than his predecessors. By 1292, he had either given up or was forced to give up--it's not clear. A ruler probably handpicked by Nogaj Khan (that would be Theodor's father-in-law) ascended to the Bulgarian throne. George fled to Byzantium, where he was apparently forced to live in a situation far below what he perceived as his station.

But new ruler Smilec couldn't tame Bulgaria--much less his own family. A succession crisis between his brothers and his wife as regent for their son (she won) gave George's son Theodor, master of the long game, his opening. In 1299-1300, Theodor leaned on his brother-in-law Chaka Khan to go after the Bulgarian throne with the might of the Golden Horde. He didn't have to work too hard. Chaka's father Nogai was busy losing his power, territory, reputation, and why not also his life to onetime ally, now bitter enemy Toqta Khan (himself the husband of one of Emperor Andronikos' daughters, named Maria, because WHY NOT). Chaka needed an outlet for his anger and, you know, someplace new to govern.

Wheeling and dealing, Theodor got the outsider Chaka grudgingly accepted as Bulgarian ruler. But Toqta's consolidation of power and territory changed the geopolitical calculus, and his probable conversion to Christianity couldn't have hurt. Or maybe the Bulgarian had been plotting something all along. Theodor deposed Chaka, ordered his assassination, and--finally, finally--seized the Bulgarian throne for himself.

To thank his father for the the Y chromosome donation, Theodor negotiated his release and return from exile/captivity/shame in Byzantium. To thank his father for everything else, he basically threw his dad into really nice jail.

And because all of this started with Byzantium having a little Bulgarian problem, Bulgarian tsar Theodor married Byzantine princess Theodora in 1308.

And somewhere along the way he has his minions throw Patriarch Joakim off a cliff, but no one is entirely sure why.