My advice, if you have trouble gasping the concept, do not include it. It's not like gods are obligatory in fantasy games (also they can be "assumed" or "barely mentioned" like in most RPGs (where you just have temples and clerics and that's it, no details)).

BTW, read classic mythology (greek or roman).

and the other way is that they are assended from ordinary creatures who did extraordinary deeds.

Don't do it... a whole pantheon can't be made of Herculeses :) It's "unrealistic" :D

I can present them, one of the ways is that they will be manifestations of concepts from inteligent creatures

And don't do it either :) Manifestation of concepts is definitelly a reneisance thing (or even later), not fitting medievalish fantasy.

Trudi Canavan wrote a series of books I think called the Age of 5 trilogy, which is about a war between two religious fundamentalist countries. Spoiler Alert and in that series the gods have their followers hunt down any immortals because the gods themselves were once immortals and any person who is powerful enough can figure out how to become immortal and any immortal strong in enough can become a god if they figure out how.

You could take that approach the gods in your game are living entities but the player go pursue the path to godhood but doing so will but them at the odds with the gods as they are greedy and jealous.

Trudi Canavan wrote a series of books I think called the Age of 5 trilogy, which is about a war between two religious fundamentalist countries. Spoiler Alert and in that series the gods have their followers hunt down any immortals because the gods themselves were once immortals and any person who is powerful enough can figure out how to become immortal and any immortal strong in enough can become a god if they figure out how.

You could take that approach the gods in your game are living entities but the player go pursue the path to godhood but doing so will but them at the odds with the gods as they are greedy and jealous.

Oh man i love those books :D

But yeah, i was gonna suggest the same thing. Stargate SG-1 in season 8-10 did a similar thing. You had "gods" because people who worshipped them didn't understand what exactly they are. And they were ascended beings that once were ordinary people who found some kind of inner peace and understood the state of mind required to discard their mortal flesh and go to a higher state/plane of existance. So basically, they have all the characteristics of people (so you can make them be as black/white/grey as you want), but also have the power to influence mortals in some way (and you can decide what way that is).

If the are just manifestations of concepts or basic believes...

If they would be just people, like you and me, they would have a much deeper way...

Looks like you've already got a plan: Boss Mobs, like in Diablo and Torchlight.

I like the idea of people becoming gods. It could be a good start for a RPG-Game :)

"At the beginning there was no god, but one day a man become a hero, then a god. The first One was born."

And of course, the player is this One, and as to fight for preserving his godhood/becoming a man again/protect the world from other gods. And then : Fight-Cataclysm-Aeirth nooo!-Fight again-Save the world, happy end.

Hum.