I guess the first gam can hit my nerves, The Library always gets me so frustrated at how long you have to hold out for, the last two levels set you against 3 diferent groups that you seem to be in the middle of, that generally require a number of differen strategies. The Flood creep me out, true to their name hat just keep going and going, and some can have some rather dangeros weapons that are not obvious at first look. I guess two betrayals also feels very chaotic, while earlier you could break ito sleeping groups they are all out for blood.
I would say that the level Cortana in the third alo leaves me stressed, the levels are hard to see in, and they include the pure forms that makes long range a danger, or much risk in getting closer.
I think a similar feel also put into their teams in Halo Wars, luckilly no flying Flood have appared in other games.
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Through dream I travel, at lantern's call
To consume the flames of a kingdom's fall