Luthor Tyrell

"My husband, the late Lord Luthor. He managed to ride off a cliff whilst hawking. They say he was looking up the sky and paid no mind to where his horse was taking him."

- Lady Olenna Tyrell

Luthor Tyrell is an unseen character in Game of Thrones. He is deceased when the events of the series begin and is not expected to appear in the series. He was mentioned by his widow Olenna Tyrell in "Dark Wings, Dark Words".

Background
Luthor Tyrell is the father of Mace Tyrell, the Lord of Highgarden and the head of House Tyrell. He is survived by his widow Olenna Tyrell, known as the "Queen of Thorns".

According to Olenna, Luthor had twitchy little ferret's face and ludicrous silver hair.

In his youth, Luthor was expected to marry Viola Redwyne. The night before he was due to propose to her, however, Viola's sister Olenna stumbled upon his bedchamber after getting lost after her embroidery lesson. The next morning, Luthor did not propose to Viola; in fact, he never left his chamber, as he couldn't walk, and could only think of what Olenna had given him the night before.

Season 3
Lord Luthor's widow, Lady Olenna, describes her late husband to Sansa Stark as "an oaf". She explains that her husband died when he rode over a cliff while hawking. He had been looking up and not paying attention to where his horse was going. She used the incident as a metaphor for the Tyrells' alliance with the Lannisters.

In the books
In the A Song of Ice and Fire novels, Luthor Tyrell's backstory remains the same.

After short conversation with Olenna, Tyrion Lannister wondered if Luthor deliberately rode off that cliff.