With the new epic content in MoTU, the spell no longer functions on most new enemies that are above 30 hit dice, and there are quite a few of them. The spell needs reworked anyway, and now is as good a time as any to do it. Currently there are three 'versions' of this, slightly wonky spell.
Trap the Soul - 10 HD
Trap the Soul - 20 HD
Trap the Soul - 30 HD
I recommend scrapping all of the above and creating a new spell, 'Trap the Soul'. The caster need no longer select a specific version, the game does it automatically AND utilizes the correct and most efficient shard from your pack. I feel this was the correct way to implement to begin with.
In addition, without changing too much about the curent system, three new shards are added to the current set
Tiny Khyber Dragonshard - up to 10 HD
Small Khyber Dragonshard - up to 20 HD
Average Khyber Dragonshard - up to 30 HD
NEW - Greater Khyber Dragonshard - up to 40 HD
NEW - Superior Khyber Dragonshard - up to 50 HD
NEW - Epic Khyber Dragonshard - above 50 HD
The new shards are acquired same as before, either by drops or combining lesser dragonshards in the Stone of Change.
Naturally this opens up a lot of other things down the road like new crafting recipes and equipment, new bane-type weapons, etc.