World changes are a game mechanic that can cause changes to the Tibian world. For example, a world change might pose a hazard for nature (Thornfire) or may soak up water in river streams (Awash). Some changes can only be applied after the server save, some are reset back to their initial state after the next one, while others may be shaped by players action and last more (Sea Serpent) or less (Master's Voice) than a day.

You can check the progress of any world change by asking a Guide NPC, using any of the keywords: Horestis, Mage Tower, Master's Voice, Swamp Fever, Thornfire, Twisted Waters, Awash, Steamship, Horses, Overhunting, Demon War, Sea Serpent, Deepling, or Hive
(official announcement here).

Name Rec. Level Prem Location Reward
Awash World Change 25 Kazordoon Until the next server save, for all players on the server: access to Deepling Scout mines; ability to travel from Kazordoon to Thais and Robson's Isle. Possibility to kill Groam and get the Eye of the Deep achievement.
Deeplings World Change 130 Quirefang Access to more areas.
Demon Wars World Change 100 Edron (Hero Cave) Chance to obtain the achievements Shaburak Nemesis and Askarak Nemesis
Hive Born World Change 150 Quirefang. Access to the inner Hive Born, possibility to complete the Insectoid Outfits Quest and also to tame a Ladybug.
Horse Station World Change 8 East of Thais, West of Venore. Ability to rent Horses (applies to entire server, even free account players). Also during the world change, War Horses may be tamed.
Insectoid Invasion World Change 20 Greenshore ?
Overhunting World Change 20 Forests around Ab'Dendriel and near Carlin and the Orc Fortress White Deer can be mounted when they are not overhunted.
Steamship World Change 60 Thais Until the next server save: ability to travel from Thais to Kazordoon; access to Robson's Isle.
Swamp Fever World Change 8 Venore. Potential to obtain Slug Drug and some strange pieces of cloth
The Fire-Feathered Serpent World Change 100 Seacrest Grounds Chance to complete the Twenty Miles Beneath the Sea Quest and get the achievements Mind the Step! and Snake Charmer, ability to fight Renegade Quaras.
The Mage's Tower World Change 100 Zao Steppe, another dimension Raging Mage, Yielothax loot. Access to Uniwheel (Mount).
The Mummy's Curse World Change 100 Horestis Tomb near Ankrahmun. Access to Sandstone Scorpion for mounting.
Their Master's Voice World Change 80+ Edron. Potential to loot items for Elementalist Outfit, and a Golden Can of Oil.
Thornfire World Change 30 Shadowthorn Crystal Wolf (Mount), achievement Firefighter, Firestarter loot: Coal, Flintstone
Twisted Waters World Change 20 Lake Equivocolao in Tiquanda, north of Port Hope. Potential to catch Shimmer Swimmer, Biodegradable achievement.