Immortal Creatures will not die of old age. Or, rather, they will just live for such a long time that you'll never see them die, unless you leave your game on forever and you can stay alive.
A few genomes intentionally create immortals (often ones who cannot breed, to prevent the world from being flooded), but it can also be caused by genetic mutations, such as:
- a large amount of glycogen produced (see Highlander). This is a C1 only method!
- decreasing the half-life of ageing/life
- changing a chemical reaction to produce ageing/life
- the time's arrow (C2) or time line (C3/DS) organs being too damaged to function
- deleting the receptor to change life stages - the norn becomes 'stuck' in a lifestage
- deleting the automatic death gene altogether
Note that the original C1 creatures do not have a gene that killed them automatically, and so are potentially immortal by default - they just tend to die in other, more interesting ways. Tarlia's updated genomes fix this issue.
Many players find immortal creatures undesirable, since they can take over a world if they can still breed. The CFE genomes try to prevent creatures from being immortal.