Plunderseeds

Plunderseeds are the name of an evidently-unnatural plant created by the chaotic magic of Discord during his reign over Equestria. The seeds were intended for use against Princess Celestia, Princess Luna, and the Tree of Harmony. Due to the Tree of Harmony's magic, the seeds did not work as Discord had planned, waiting over a thousand years before sprouting long after he had been defeated and supposedly reformed.

Depiction in the series
When they germinate, plunderseeds create a quick-growing and extremely tough plant. In their normal form, they are a thick, black thorny vine without any visible foliage. They also sprout sharp, blue-white thorns. These vines are motile and apparently carnivorous, attacking ponies and attempting to crush them or consume them outright. They are resistant to Earth pony strength and spread very quickly. They grow fast enough to ensnare flying Pegasi and Alicorns, and will attempt to completely encase ponies they capture.

Inside the Everfree Forest, the Mane Six encounter another part of this growth in the form of large growths reminiscent of the Venus flytrap. As well as the obvious trap-jaws, this growth also has the ability to produce clouds of spores that have soporific and magic-disarranging effects, causing unicorn magic to go haywire in areas contaminated by the spores. The spores, if used directly on a pony, will dramatically weaken them, leaving them vulnerable to the vines' tangling and crushing attacks. The organism seems to be aware of its surroundings to the point where it is capable of an organized defense against the Mane Six's attempts to remove its growths from the Tree of Harmony.

It is clear that Discord designed the vines to be a perfect counter to all pony abilities, creating weather that Pegasi cannot control, destroying agricultural land, resisting all Earth pony efforts to destroy them, and rendering unicorn magic largely useless (and, indeed, dangerous to use). It is unclear whether Discord intended this simply to counter the power of the Alicorns, who have the abilities of all three of the terrestrial pony races, or whether he intended for them to be an attack on ponies in general.

Strengths and vulnerabilities
The vine growth is immensely tough and strong. It cannot be uprooted or torn by Earth pony strength. It is also fast-growing enough to outmaneuver and trap ponies, even fast-moving and airborne Pegasi. The spores are shown to be powerful enough to overwhelm an Alicorn, and their magic-disarranging effects are highly effective in rendering unicorn magic functionally useless. It is thus shown to be a purpose-designed weapon intended to render the three pony races completely helpless.

While the vines show some ability to counter intelligent action, they seem to mostly react in a series of pre-programmed instinctive responses to capture and destroy their targets. It is possible to fool the plant into tangling itself and render it unable to move. Additionally, the direct application of blasting unicorn magic is shown to destroy the growths (although, as this attack was from an Alicorn, it is possible that an ordinary unicorn would be insufficiently powerful).

The growths are unable to resist the full power of the Tree of Harmony and are completely destroyed within a minute of the Tree regaining its full power.

Princess Twilight Sparkle - Part 1 and Part 2
The plunderseeds are the primary threat encountered by the characters in the season four premiere.

Two days before the year's Summer Sun Celebration, the seeds germinate and begin to carry out the function for which Discord designed them over a millennium previously. Their first act is to capture Princess Celestia and Princess Luna, causing both the sun and moon to be in the sky at the same time. It is not immediately revealed, but they also drain the energy of the Tree of Harmony, allowing them to quickly infest the Everfree Forest and grow into the surrounding area, notably Ponyville. The plants quickly overwhelm Sweet Apple Acres, evidently being immune to Earth pony strength and nature magic. Soon, the growth completely overwhelm Ponyville, resisting all normal pony methods to control nature and demonstrating clear malevolent intent by turning unicorn magic into a serious threat to its casters and those around them and attempting to drive Pegasi out of the air.

During the premiere, Fluttershy, Twilight Sparkle and Pinkie Pie all have to be rescued from being snared by vines or their trap-jaw growths. Cherry Berry and Comet Tail are seen being crushed by vines and are only saved when Discord, cued by Twilight, reluctantly uses his chaos power to wipe that particular growth from existence.

The core of the plunderseed growth is found to be around the Tree of Harmony. The Mane Six prove unable to defeat the growths until Twilight realizes that the Tree itself needs to be recharged by returning the Elements of Harmony to their rightful place. Once this happens, the magic of the Tree disintegrates the growths all over the affected area (much to Discord's disappointment) and frees Celestia and Luna from their prison within the growth's core.