Modify > Fillet > Fillet Two Lines

The Fillet tool creates an arc or radius between 2 lines, circles or arcs to a specific radius. In the example below, 2 diagonal points are selected. These points can be selected in any order.

Selecting diagonal points for the blend

A radius of 15 is input into the Radius field. The result is shown below.

Fillet blend of 15 created