Shadow Blades is a level 72 Subtlety rogue ability that significantly increases damage and combo point generation.
The damage increase comes from the fact that, as pure shadow damage, auto-attacks will ignore armor for the duration of the effect. It is quite effective to use in concert with many other cooldowns. Energy pooling before using Shadow Blades can help make the most of the increased combo point generation.
Patch 7.0.3 (2016-07-19): Re-added. Now lasts for 15 seconds, up from 12.