Elder Princess Shroob

"Foolish mushroom heads...You may have bested me, but Shroobs will live on...One day, the Shroobs will dance on the buried remains of this pathetic kingdom!"

- Elder Princess Shroob

The Elder Princess Shroob is the elder sister of Princess Shroob and the final boss of Mario and Luigi: Partners in Time.

Mario & Luigi: Partners in Time
Princess Peach enters a time machine designed by professor E. Gadd and ventures into the past- at the same time, The Shroobs captured the Mushroom Castle of the past. She is then faced by both Princess Shroobs. In desperation, she grabs the time machine's power source, the cobalt star, and manages to trap the Elder Princess Shroob's essence within it. Peach is then captured by the younger Princess Shroob.

During the events of the game, Mario and Luigi believe that they should gather the shards of the cobalt star to be able to rescue Peach. The Princess Shroob contacts the Mario Bros. and pretends to be the spirit that resides within the star, telling them that if they gathered the shards, then they could defeat the Shroobs.

After the Mario Bros. kill the younger Princess Shroob, Baby Bowser drops into the room, steals the cobalt star, and puts it together, which releases the Elder Princess Shroob. Mario and Luigi defeat the Elder Princess Shroob with some assistance from Peach. But then, she taps into the power in her crown, and mutates, showing her true form, which is so horrific, Peach collapses. Mario and Luigi then have to defeat her by attacking her crown, which made her nearly invincible, then attacking the Elder Princess Shroob herself. After a long, difficult battle, Mario and Luigi kill her. In her dying breath, she swears that the Shroobs will have vengeance, and destroy the Mushroom Kingdom. She turns into a small purple mushroom, which Baby Luigi picks up.

When the Mario Bros. arrive at the Mushroom Kingdom of the present, they find Bowser unconscious. The mushroom that was once the Elder Princess Shroob floats out of Baby Luigi's pocket, and into Bowser's mouth, turning him into Shrowser. The Mario Bros. defeated Shrowser, destroying Elder Princess Shroob for good and releasing Bowser.

Mario & Luigi: Bowser's Inside Story
During the optional battle with three Shroobs in Bowser's Castle, several Shroobs can be seen frozen in the background, including Elder Princess Shroob. This has given way to much confusion, as she was clearly destroyed at the conclusion of Partners in Time. There are many theories behind this:

Or perhaps all the frozen Shroobs in the battle's background weren't really there (as, outside the battle, the only other Shroob in the room besides the three Mario and Luigi fight is Junior Shrooboid, who never seems to be destroyed in Partners in Time), and were a battle background decoration to remind the players of Partners in Time. This is most likely given that outside of the battle the room is too small to have such large amounts of frozen Shroobs lined up as a crowd.
 * The Shroobs first attacked Mushroom Kingdom in the present (sometime before the events of Bowser's Inside Story), but ended up attacking Bowser's castle first, and Bowser froze them. They were then rescued by Princess Shroob, and Elder Princess Shroob (who has been established as being able to create time warps) traveled into the past to take over Mushroom Kingdom there, easier, when Bowser was Baby Bowser. (though it is implied in the game that Bowser froze Shroob survivors such as Junior Shrooboid).
 * Partners in Time took place AFTER Bowser's Inside Story. (This idea has often been rejected due to Fawful's death at the end of Bowser's Inside Story, and his appearance in Partners in Time).
 * The surviving Shroobs cloned or revived her.
 * The frozen Elder Princess Shroob is really a sculpture, or a painting on the wall.
 * Mario and Luigi's time traveling altered the flow of time and changed the present.
 * She unbelievably managed to survive the battle, despite being turned into a mushroom, eaten by Bowser, turning into a ghost and possessing him, and being destroyed without a trace.
 * Her cameo was just a nod to people who may have played Partners in Time and recognized her.

Quotes
"I fear nothing now! Do you hear me? Nothing!"

- Elder Princess Shroob

"I will give you a special reward for setting me free. You will love it."

- Elder Princess Shroob

"I LAUGH AT YOU, MUSHROOM HEADS. PATHETIC THINGS!"

- Elder Princess Shroob taunting the Toads.

"And my sister...My dear, sweet, beautiful sister...I WILL AVENGE HER!"

- Elder Princess Shroob swearing to avenge her sister, the Princess Shroob.

Trivia

 * Elder Princess Shroob is similar to Dark Bowser, a final boss in Bowser's Inside Story, as both are killed directly by Bowser and indirectly by the Mario Bros (though in Elder Princess Shroob's case this wasn't intentional on Bowser's part). The way she explodes into purple particles is also repeated by Fawful, who serves as the other final boss of Bowser's Inside Story.
 * Gloomtail is also similar to Elder Princess Shroob as both wanted to avenge their sisters' deaths.