Complex sentence: Having attained a position of influence, William Graham initiated a suit to try and reclaim the earldom of Strathearn (a land that encompassed all of Menteith and others) that had been lost by Malise Graham, 1st Earl of Menteith, in 1427.
Split into simpler sentences: Having attained a position of influence, William Graham initiated a suit to try and reclaim the earldom of Strathearn, comprising the lands of Strathearn which encompassed all of Menteith and other areas. This earldom had been lost by his ancestor Malise Graham, 1st Earl of Menteith, in 1427.