In the Malazan Book of the Fallen series, it is implied people 'ascend' to become gods (or god-like)? What decides who ascends and how does it work?
It seems that a being needs an enormous amount of power to be able to ascend OR like Kellanved and Cotillion they can take the unoccupied throne of a Warren to become a god. Ascension seems to mean that age doesnt kill someone they just keep on going like the energiser bunny.
The difference between gods and ascendants is that though both are immortal (everyone is killable though, there arent any true immortals) ascendants have no worshippers, or refuse to accept worshippers. Annomander Rake is the main example of this, he is worshipped as a god by the Bluerose Tiste Andii but long ago refused their worship, though how that works....
First let me deal with the ascendancy god issue. All gods are Ascendant but not all Ascendant are gods. An ascendant are any body who cannot die from natural causes (ie. old age, sickness). They can be killed by normal means and have access to magic (either amplified from their pre-ascension or new). ( new -ie. not elder )Gods are ascendant that have acquired control over a warren - via the power of a Throne. There is not much actual difference between (new) Gods and Ascendant. From a technical perspective all of the new gods were at one point ascendants (Hood won a throne, Shadowthrone the same). Note that they merely had to gain power via a throne - this is how Cottilion is a god although he isn't the ruler of Shadow. Elder Gods were born gods and have to be 'powered' by blood sacrifices.
If you get the power of a Throne you ascend into Godhood. Generally speaking Ascension occurs after the person has experienced a (unspecified I think) amount of power from a Warren/realm/hold whatever suits you. Gesler ascended via the fires as an example of this type - he resisted ascending for some time and the books may/may not (not really clear) have ended with him not ascending but it's clear that the fires were the cause of the near ascension. Also being chosen into a House of the Deck of Dragons is hinted at being a sign of ascendancy as nearly everyone who is mentioned in it is either recognized or implied as being ascended. There are a few exceptional cases that are slightly obvious that I won't go into (eg. Sorry).
Hope this helped, I know how confusing Malazan can be...
Still there are some quite strange ways of being ascended. For example how whole company of bridge burners (sorry but I have read book in diffrent language than English and it's hard for me to find best translations), they ascended either by the canto/song that was used by some kind of "elder" (it was somehow related to Fiddler/Violinist/String). Or it may be my mistake and they ascended thanks to the Hood? (the god of dead).
Second strange example is being chosen as an avatar of ascended one. Here for exaple I can bring Grumbler, avatar of "tiger of summer". His incomplete ascension began after he wore ring on his arm. Then the changes related to summer tiger apeared and Grumbler got tiger stripes on his body, he became stronger, faster, etc.
So we may assume that creature have to get in contact with god (like god of war tiger of the summer) and get chosen as his avatar. This will result in becoming ascended creature or at least ascended-like creature.
Or creature can find an empty throne then take the ascending power from it.
Or the creature may come in contact with an item/artefact related to the ascended god-like being.
Well I think there was also a possibility to become ascended like creature by consuming part of ascended one being or getting some kind of creature into itself like Locui vywal/wraith(can't remember their names now, sorry) or god like Cottilion.
I don't really understand how Geslers company ascender in grotto. It's like they ascended through sheer will power and by staying alive in the grotto of fire? Like @Ender Delat mentioned earlier.
Sorry for my English. But I wanted to try sharing my knowledge. Still I may be wrong but I hope that at least I brought some of the usefull examples that may bring some light to this topic.
I agree with the answer above, I just wanted to add further examples of Ascendency:
Ascendency via song:
Fiddler saved a little girl from muggers somewhere in Seven Cities. That little girl's family was headed by Kimloc, which that book implies was one of the more powerful Tanno Spiritwalkers. The song was sung into a shell, which was later broken in a battle, as a shaved knuckle(knuckles being a Malazan soldier game of chance, and shaved ones your ace in the sleeve way to cheat). That break awoke the Tanno song, calling back all the dead Bridgeburners to help win the day. This act caused the ascendency of the Bridgeburners.
Ascendency via deep trauma:
Gruntle's Ascendency is complicated. When GotM starts, Fener, the Boar, is god of War. Trake, the Tiger, is a First Hero, a Soletaken, and an Ascendent, but all assume he's lost his ability to semble back into human form. Through a bunch of plot I won't spoil, Trake rises in prominence and picks Gruntle as his mortal sword. Gruntle undergoes his transformation in a horrendous siege occuring in the Pannion Domain war. >!
Ascendency via compassion:
Finally, one last ascendency should be mentioned. Itkovian, the Grey Swords Shield Anvil, ascends by assuming a massive burden. >!
Here is a quote from House of Chains on one character's musing on the subject.
Crokus had no idea how such a thing could come to be. Ascendancy was but one of the countless mysteries of the world, a world where uncertainty ruled all--god and mortal alike--and its rules were impenetrable. But it seemed to him, to ascend was also to surrender.
Embracing what to all intents and purposes could be called immortality, was, he had begun to believe, presaged by a turning away. [...] was it not a mortal's fate, then, to embrace life itself, as one would a lover? Life, with, all its fraught, momentary fragility.
And could life not be called a mortal's first lover? A lover whose embrace was then rejected in that fiery crucible of ascendancy?
Well it is a thematic extension of the leveling up concept, so the ascended seem to be those who have accomplished great deeds and made a name for themselves/drawn the attention of powers (or reached level 20), admittedly I'm on part way through the second book at present so might be missing something...
This is backed up by the section of Deadhouse Gates where the half Jagg and the other guy are in the desert with the priest of Shadow and he says something along the lines of "Why are you too still adventuring when you could Ascend" - this heavily implies that it is a personal choice and the means to ascension just become available upon "reaching level 20".