The Valley of Kings is the name of two different but very similar decisions regarding the excavation of The grave of Tutankhamun. The first and most common one requires can be taken by any great power, if 
Egypt still owns Luxor and is not in the sphere of said great power. The other one can be taken by any civilized nation, which owns Luxor including a civilized Egypt, or any great power, which has Egypt in the sphere of influence.
Option 1 - An uncivilized Egypt owns Luxor
It is available once a Great power has researched Nationalism and Imperialism and have at least 25 prestige. Egypt must exist and own the province of Luxor (ID 1767). Additionally Egypt must NOT be sphered by the nation in question.
It gives Egypt the event Excavation Rights Requested, where it can choose wether to allow the excavation or not. Additionally the great power in question will get the modifier 'international pressure', giving 0.1 infamy pr. month, until Egypt accepts or at most for two years.
If Egypt allows the excavation, the Great power gets the event Egypt Grants Excavation Rights, which gives 5 prestige, as well as an additional 0.01 prestige pr. month while the excavation unfolds. It also improves the relation between Egypt and the great power in question by 20. On the other hand, if Egypt rejects the excavation with Egypt Denies Excavation Rights, the great power loses 5 prestige and the relation between the two nations are lowered by 20.
An AI-controlled Egypt has 30% chance of accepting the proposal. This is tripled to 90% if the asking nation has more than 65 prestige.
Option 2 - Luxor is owned by the nation, taking the decision
Alternatively, any civilized nation that owns or have sphered the owner of Luxor, including a potentially civilized Egypt, can excavate the Valley on its own. This time around, one doesn't have to negotiate with Egypt and the entire event chain regarding the acceptance or denying of excavation is bypassed. The decision simply gives 5 prestige.
Futher complications
After the excavation begins, either by option 1 or 2, a long list of possible events can happen, with a long list of possible effects both positive and negative. They can give prestige and research points if a grave is found, give militancy, hurt relations with Egypt or demand further funding to name a few.
If the crownjewel of the excavation, Tukankhamons Tomb, is found it gives a staggering 1 year worth of research points as well as 30 prestige. It does however have a mean time to happen of 2000 months(166,7 years).