You get a soul shard from any mob which yields experience when defeated. Turn on "drain life" when the mob is down to about 25% of its life, and as long as it dies with that spell on, you will see upon its death in your chat window that you receive a soul shard. The mob does not have to be humanoid, as my warlock has filled her bag with shards received from plainstriders, lions, and raptors, which are NOT humanoid. But when you select the mob, if its level number is grey, you will not get a shard. I also think that if you are pvping with this, if you receive honor from the kill with this spell, you receive a shard in that situation.