The damage reduction is 15 over 5 turns, so roughly equivalent to a 5-turn 3 pt. shield. It is weaker than that though because the number of enemy creatures tends to increase with the turn count. Still, it is interesting because you can chain it. If you want to, you can chain it every 3 turns and have a steady ~ 4 pt. damage reduction. Or chain it every 2 turns for ~ 4.5. Over all, it is an interesting effect. It might be even more interesting if it was 2*X + 1.