Mumbai beat Uttarakhand by 725 runs in the Ranji quarter-final which concluded today:
This is the highest margin of defeat by runs in all first-class matches, surpassing the 685 runs margin for NSW v Qld in 1929-30 at Sydney. The existing records can be seen here:
Note that the 675-run margin for England vs Aus in 1928-29 at Brisbane is next on the list.
It can be seen that the Mumbai team included only two (fringe) international players in Prithvi Shaw and Dhawal Kulkarni. The main scorers and wicket-takers are relatively unknown.
In fact, the cricket grounds at Alur are situated at a rather remote part of greater Bengaluru, near Chikkabanavara: