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can 2 monolith be used in the same army list?
10x man , a had 2 monoliths and my opponents stated that it was against the rule.
As avatar of khaine has stated, you can field up to three Monoliths in a regular game of 40k, assuming your points limit allows it. Check out the Force Organisation chart in the 40k rulebook, page 87. As long as a unit/model doesn't have a "0-1" limitation next to it in your codex, or the "unique" special rule (usually seen with special characters, such as in the Eldar and CSM codices), or simply has a rule stating you can only take one (such as the C'tan), then you should feel free to take as many as you like as long as your choices conform to the Force Organisation (given you aren't playing Apocalypse).
But as stated above, there isn't anything that disallows 2 Monoliths in a standard game, except point cost.
Yeah one HQ and 2 10 mans of warriors are required before fielding other things first. But one thing I've noticed, if I field multiple monoliths, or 15 destroyers, or crap like that (especially in a casual game), you'll find your willing opponents tend to drift away (unless of course they beat ya...)
"There is only do, or do not. There is no try." - Yoda
As a 'Nid player I hit that too. Noone wants to play 500pts with me anymore. Don't like 4 MC running around killing things. So far I haven't seen anything in my group that would encourage someone to bring more than one Monolith but it is perfectly legal as long as you have the points for it.