This as a composite proposal of my own ideas, as well as those entertained by Barlth, Artur, and other members of the community, about the implementation of the Gods.
Emissaries: If the player goes around offending gods they will send their emissaries (like angels) against them in the mortal realm. For instance, defiling the dead (practicing necromancy) would offend the God of The Dead, and they might periodically send wraith-like emissaries against the player.
Challenging the Gods: Each god could have a temple dedicated to them, located in its own distinct region of the world. The concept of a “temple” could be used loosely for more unique environments, such as temple to the God of The Dead being a labyrinthine catacomb, or an entire island acting as the temple to the God of The Sea.
Worshipers protecting the temples would have their own unique resistances, weaknesses, and abilities depending on which god they serve. Each temple would have an altar, and if you can cleave your way through the protectors you could use the altar to challenge the god to come to the mortal world and fight to the death.
Optional Weaknesses: Each god could have a unique weakness, but not one the player could readily exploit. For one god, the player might go on a quest to recover an artifact that would give them an advantage when they fight that specific god, for a different god the player might seek to gain the assistance of long-since banished mythical beast by releasing it into the world to help them defeat the god that banished it.
These weaknesses could be used to gain an edge in battle, but wouldn't be necessary to defeat each god and would by no means guarantee victory either. What it would do however, is pad the game with meaningful content and flesh out the lore/history of each god for those who decided to opt for it.
Consequences: Each god would preside over a certain aspect of the world, and their death would cause that aspect to fall into decay; such as the rivers progressively drying up over time after killing the God of The Sea. In this way, the player would need a strategy for killing the gods in a certain order based on their needs. Once they kill the first god, it would essentially be a race against time to kill the rest of them before the player can no longer sustain the life of their mortal embodiment, in an effort to bring down the entire world with themselves.
Sea God – Rivers slowly start to dry up over time.
Sun God – The sun grows dimmer with each passing day, until the day is as dark as night.
Forest God – Plants and wildlife start to die off, grass turns to dirt, trees and bushes start to disappear.
Death God – The dead slowly start reanimating, unable to crossover to the afterlife.
Mountain God – Ores start turning to stone, stone begins to crumble to gravel.
Sky God – Lightning storms become progressively more frequent and violent, potentially setting fire to tree/buildings if that is implemented.
Each God would also naturally preside over it's emissaries, for the Death God it could be wraiths, for the Sun/Fire God it might be fire dragons. In any case, killing a god would forcibly scatter their emissaries across the mortal world, like roaming mini-bosses that would make the world progressively more dangerous and inhospitable.