How best to repair a cavity in the base of a retaining wall
Wow...there’s a lot of issues: 1) height, 2) non-reinforced wall, 3) load bearing, 4) moisture 

1) This is a tall wall at 10’ or so. That means if it is a true retaining wall, it has a lot of stress on it if it’s load bearing, moisture bleeding through the wall, etc. 

2) This is a “gravity retaining wall”. That is to say it’s a non-reinforced wall. Non-reinforced walls that high generally need to be quite thick (heavy) at the bottom...like 3’ - 4’ thick...or the interior of the wall is poured in place concrete with reinforcement. 

3) If the wall is load bearing, it complicates the repair, because it needs to be sturdy and permanent and may need to transfer seismic loads or high wind resistance. 

4) If there is substantial water coming through the wall, the wall needs to be strong enough to resist the water pressure or be designed to divert the water or let the water pass through the wall and then be diverted. 

If water is allowed to pass through the wall, it must be controlled so it doesn’t undermine the footings and/or stay in the crawl space. 

So, before You start, I’d verify that it’s a non-reinforced wall so the repair does not need to be reinforced, verify if moisture should be allowed to pass through wall so it can be controlled, etc. If those things apply, I’d clean all surfaces to receive mortar, use exterior grade mortar...probably a Type M mortar, then carefully layout the pieces so the can fit (or be cut to fit) the opening. Do not leave voids in the opening. Completely fill the void with slate material or mortar.