Hollow concrete clock can be used for a retaining wall in some circumstances. One of the most important concerns would be, how high is the wall going to be?

As there is nothing to hold the blocks together in order to work as one unit, as you would find in a poured-in-place concrete wall, or even the modular concrete block walls that are popular today, you should plan to fill all the voids in the blocks with concrete. You also should install some steel reinforcing rods, both vertically and horizontally, before filling with concrete. If you've never done this before, and it sounds like you haven't, you shoul get some local expert advice on how to do it.

As for drainage behind the wall, once again, what you install will depend on how high the wall is. The base material you place (and compact) under the wall should be free-draining, like crushed stone. But that alone will not take care of drainage behind the wall. Remember that a retaining wall is essentially a dam for ground water. Weeps through the wall, (1/12 to 2" pipe), installed every 10 feet or so will assist in getting water out from behind, and will have to be cored through the wall and screened at the backside to prevent them from becoming filled with sediment. Most often, a perforated PVC pipe is installed at the backside of the wall, equal to or slightly higher than the visible grade on the front side. This pipe runs from end to end of the wall and daylights (sticks out of the slope) or connects to a drainage system to remove the water. The pipe is bedded in and surrounded by a free-draining material, like crushed stone, and the whole thing (stone and pipe) may be wrapped in a filter fabric to keep sediment from clogging the pipe.

A retaining wall project can be satisfying, but it can also be a nightmare. Best to seek local advice and 'how-to' from someone who might be able to give you some pointers personally.

Good luck.