Signet of Capture

Use a Signet of Capture on a dead boss. If you choose to learn one of his skills, Signet of Capture is permanently replaced by that skill. (You can only learn skills of your current professions.)

Where To Get It
Magi Malaquire in Lion's Arch gives you the quest Malaquire's Test and one Signet of Capture upfront, and another one as part of the reward. Thereafter, you can purchase additional signets from him or most other skill trainers. Alternatively, you can purchase it (regardless of completed quests) at the skill trainer in Quarrel Falls, the Amnoon Oasis, or Port Sledge. Apart from the obligatory skill point, Signets of Capture cost exactly half of what buying a normal skill from the skill trainer would cost.

You can possess more than one at a time.

Usage
First, before entering a mission or explorable are, equip as many Signets as you wish. Although every Signet of Capture does the exact same thing and is identical in every way to any other Signet of Capture, technically they are different skills, so they do not violate the rule that every skill can only be equipped once.

To use the Signet of Capture, defeat an enemy boss and then invoke the signet near its corpse. You will see a dialog box listing all dead bosses within reach (if there is more than one) and for each one, it will list all the skills this boss has and which you can capture (Elite Skills and regular skills). Click on the one you wish to capture, confirm, and the signet will be replaced by that skill. If you see nothing you like you can cancel the capture and keep the signet for a different boss.

Once a skill is captured, Signet of Capture will be permanently replaced by the captured skill in your skills list. A new Signet of Capture can be purchased to capture further skills.

Using a Signet of Capture is the only way to learn elite skills in PvE play. This signet can only be used near dead bosses, and you can only capture skills of your primary or secondary profession.