Excalibur

Excalibur is a legendary sword holding the ultimate power. Only the true master has the power to wield it fully and completely. Whoever wields this sword will become invincible.

In legend, it is the sword given to King Arthur by the Lady of the Lake. At Arthur's death, Sir Bedivere threw Excalibur into the lake; a hand rose from the water, caught the sword, and disappeared. Another legend says Excalibur was drawn from a stone by Arthur to prove his royalty.

The only two capable of holding it now are Piper and Wyatt Halliwell – its new Lady of the Lake and destined master.

It is a sword that possesses powerful magic that protects itself from danger. It is also protected from potions thrown at it and is fully capable of even vanquishing a then extremly powerful Mordaunt as well as many other demons in the episode as well.

Piper as Lady of the Lake
When the Sisters met the real Lady of the Lake, she was running from demons who wished to steal the sword from her. She was killed by a demon, and she gave her rite to protect and wield the sword to Piper. Piper didn't know that she was the current Lady of the Lake until she pulled the Sword of Excalibur from the stone that it flew into and wedged into. Paige orbed it into the Manor and the Sisters began researching about anything having to deal with the sword. Magical creatures from far and wide, came to see if they were to be the Sword's Lady of the Lake or it's destined master. Piper became frustrated with all the magical beings in her house trying to take the Sword out of the stone and she ripped it out of the Sword and became distraught that she was the new Lady of the Lake. Her mentor on controlling the Sword became corrupted at the power he could gain, and used Piper and told her to give into the Sword's pull for fighting. She gave in, and her sisters tried to protect her by destroying the Sword. When they threw the potion at the Sword, nothing happen due to it's immunity to attacks and magic. The mentor told Piper to start her own "Knights of the Round Table", but he just stole the powers of the demons they gathered.

Powers and Abilities
When someone is in control of Excalibur they are considered an Ultimate Power so they are invincible. Similar to the crystal sword the wielder of Excalibur can call it back to them and is able to teleport through whirling. The sword is immune to potions and is able to destroy even the highest level of demons. Although Piper and Mordaunt were able to control Excalibur at some point the true wielder of the sword was Wyatt and it's unclear what other abilities it might possess when in control of Wyatt since he only used it once as a toddler to vanquish Mordaunt.