Lovecraft Mythos is a survival platformer. Collect 9 keys to advance to the next screen, trying to avoid the eldritch monsters which will continuously respawn around you. Shooting a monster will kill it instantly, but be wary of the limited ammo. Try to conserve it as much as possible.
You can only survive a limited number of hits, represented by the hearts at the top left corner. Every time you clear a screen, your remaining life and ammo will be restored. In addition, the game will always start at the last screen you reached. Progress through the 8 existing screens to win honor and glory, which will be useless because the game will start again from the first screen and you'll die anyway.