No one and no animal eats a lion (provided the lion is alive) – wrong..some Americans eat lion burgers. One website states that the young Maasai are meant to kill and eat a lion (as a rite of passage). This is false today in that they hunt lions but do not eat them. They use three body parts (the mane, tail and claws) for various purposes none of which can be justified in a modern world where the lion is endangered.
Perhaps killing a lion amongst Maasai men is seen as brave and so on (a status symbol for the killer) but let’s be frank the Maasai are a backward people living in medieval times or earlier.
They firmly believe in female genital mutilation. This is barbaric and illegal (thank God) in the UK. This belief alone places them amongst the most backward culture on the planet. And let’s not be sentimental and have romantic notions and praise these sort of ancient cultures if they demand these sorts of practices. This is male domination of women; something that was present in the West many years ago.
So the answer to the question: what eats a lion? is that no animal does because the lion is at the top of the food chain unless the lion is dead and the carcass is being scavenged by vultures and hyenas and other animals.
They might be wise not to scavenge a dead lion because sometimes they have been poisoned by farmers to protect their livestock and the poison then kills the scavengers.
If there was a predator that was larger than the lion and as competent, or as large, then that predator might kill and eat the lion but such an animal does not exist.
Associated: What eats a cheetah?
Note: I have written about the African lion. The situation is different for the Asiatic lion living in India (Gir Forest). There they live on a large reserve and are protected from human predation/persecution.
Note 2: this is a second page on this topic. I have discovered that I have covered it already!