The above video and Wikipedia, below, summarize what a MegaGame is much better than I ever will. My one thought is that you will never have a true appreciation of megagaming until you have played in one.
A megagame is a type of large-scale simulation which can contain elements of role-playing games, tabletop games, LARPs and wargames – the amount of these is dependent on the scenario being played and the way the players choose to engage with the scenario. Participants can be arranged into hierarchies of teams – some games will consist of multiple competing factions and others will have teams operating in different game ‘niches’. Megagames have been played with up to 300 people,[1] though are more usually played with 30-80 players.
Wikipedia