Adaptor for 20mm Shimano saint 810 hub to 15mm?
20mm hubs have an axle width of 110mm whereas a 15mm thru axle is 100mm. This means that there is no backwards compatibility on the hub unless it was already designed that way (such as Hope 2 hubs). The M810 is simply too wide.

You would need to replace the front hub at least to run a 15mm TA fork.