Instability is the collective term for effects that differentiate unstable ages from stable ones. Instability is usually caused when symbols conflict, the player stacks symbols for material gain, or inherently unstable symbols are added.
Instability is dependent on game difficulty, players on Peaceful will have far less to worry about in unstable ages than players on Hard. (Players can override the difficulty in the config file. Unstable ages can be prevented by writing stable ages, turning off instability for a specific age via /myst-toggleworldinstability, (interchangable with /myst-twi) or turning off instability for all ages via the config file. Players can also disable specific instability effects.
Meteors are giant rocks falling from the sky, causing a massive explosion. They appear in the sky, and when they hit the ground, they explode, cause fire and, due to the explosion, leave behind a massive crater. The meteors, however, do leave behind some ores, which might make it worth the risk. Also, there is a meteor symbol which will give you the same effect, the symbol adds some stability.
Varying negative potion effects will affect the player whenever they are on the surface of the age. Blindness and Mining Fatigue effects will occur regardless of whether the player is on the surface or underground.
Decay block are blocks that alter the shape of the world. They come in different flavors: Devouring (black) and spreading (white and colored).
Black decay is the instability form first added to Mystcraft, but was renamed when the other instability mechanics where added. Black decay generates black block that are eating the world, causing columns of blocks to fall down, which is very dangerous with you in it. Black decay will open holes to the void. It spreads slowly. Can be removed easily with a shovel. Will not spread into air, however it will cause some air gaps to be filled in by falling blocks.
Different then black decay, as it won't eat the world, but convert the world into the decay block. Somewhat less dangerous, but still a force to be reckoned with. There are 3 different colors, red, blue and purple. They spread at different rates through different materials, and at a generally moderate pace -- faster in some, slower in others. Can be removed with a pick-axe; gold is recommended for any non-trivial sized infection.
Functions the same as colored decay, but does damage to all mobs that **move** (walk, run, etc) on it. Also spreads a lot faster then the colored version. Spreads too fast to be removed, but you can clear a path with a pickaxe; diamond is recommended. NB: You can heal while standing still, but it will spread as you do.
Will cause blocks to break down. Dirt breaks into sand, ores turn to stone, stone becomes gravel, etc. Will turn the age into a wasteland given enough time. \\ //It is worth noting that diamond ore turns into coal ore, which is a clever little detail that might otherwise go unnoticed.//
Hostile mobs regenerate health passively, similar to the way players do with full hunger.
Appears similar to Crumble, but appears to occur around liquids.
Explosions occur rapidly all around the age, extremely dangerous in Skylands and Void ages. The explosions are not known to damage Obsidian, (or Industrial-Craft 2 Reinforced Stone and Glass) though the explosions can occur INSIDE any Obsidian-encased buildings and destroy things inside, leaving the walls intact.
A nightmare to everyone that likes to be outside, causes lightning storms that will burn where it hits.
All mobs, including the player, catch on fire when exposed to the sky.