Dalamud Shard

The majority of the shards were left untouched after their downfall from Erozean skies, since it was assumed that the crisis surrounding those events were resolved. Why this was partially correct the entire truth was different:
 * Small cluster of shards were forming local zones of power, mostly independent from the rest, gaining a more or less autonomous state,
 * Medium and large cluster were forming a unified network,
 * Large cluster which disconnected or disconnected from the network, forming an unexpected correlation between accumulating and spending, leading to primal-like summons.

It is revealed by Leih'to Wahraindaq that despite largely visible threats (i.e. areas locked down, guarded by unusual fauna) in the closer proximity, the shards were not even suspected to be their origin. The shard's corruption is only traceable by some individuals, requiring them to be sensible to natural and corruptive aether flows – to such an extent it becomes unbearable and induces a comatose state (like witnessed during the Cleansing Pharos Sirius).

A side effect of the corruptive/negative shard aether on unsuspecting individuals is a possible subjugation similar to the subjugation process primals employ to ensure a constant flow of worshipping energy. The difference between the primal and shard subjugation lies in its immanent danger posed to the life force of the subjugated individual. This "brutal ripping out" deforms and twists the body and mind of the afflicted individual leading to its death.

The shard unified network also seemed to possess some form of artificial intelligence and sentience. This swarm intelligence disconnected from its main host spins out of control. Bereft of its host purpose, the sentient shard intelligence attempts to tap into each attainable source of aether when threatened by formidable enemies. During this desperate tapping Leih'to gains connection to the unified network and receives visions about the plans and state of the shards network: full absorption of all aether rendering the world to a lifeless place.

Origin
A not clearly specified amount of Dalamud Shards, remnants of the explosion of Dalamud – a prison designed to keep Bahamut and his dragon worshippers permanently summoned and jailed – rained down along with body parts of Bahamut (i.e. talons, scales) on multiple landing sites, devastating the landside and sometimes remaining prominently visible in the aftermath.

The widespread belief that Bahamut was defeated became a commonly accepted fact. Only a selected few knew different because of an oath of secrecy initiated by the Leveilleur Twins. Those selected few were able to witness Dalamud's recovery and activity in order to keep Bahamut alive and contained, with limited success. Eventually the situation was resolved.