Which Northern lord's secret Frey pie plot was omitted from the show?

Wyman Manderly was the Northern lord whose secret revenge against House Frey included serving Frey corpses in pies, a storyline omitted from Game of Thrones. He is the overweight but politically shrewd Lord of White Harbor and one of the North’s most important surviving nobles.

After the Red Wedding, Wyman publicly appears to submit to the Boltons and Freys. Privately, however, he remains loyal to House Stark. He has three Frey relatives killed and allegedly has them baked into meat pies that are presented during a feast. Wyman then sends Davos Seaworth to search for Rickon Stark, revealing that his apparent submission is part of a larger resistance plot.

The television series used a different version of the pie revenge: Arya Stark kills Walder Frey’s sons and serves them to him in a pie before murdering him. That scene is visually similar but is not Wyman’s book storyline. Wyman himself does appear in the show, though his conspiracy and pie plot do not.