League of Legends is its own game built on an engine Riot made from scratch.

League's Wikipedia page describes it as "inspired by Defense of the Ancients, a custom map for Warcraft III." I believe they also hired some of the core Dota devs to work on League. I guess if you want to be pedantic it was a custom map, but that was more a consequence of WC3 lacking support for mods. They ended up having to work around a lot of limitations to make Dota work in the custom map framework.

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bogdan 6 hours ago | parent [-]

They hired a former dev, guinsoo. At the time when LoL was announced dota was being developed by icefrog for a few good solid years already. Most of dota's popularity happened during icefrog's years. Icefrog later joined valve and helped create Dora 2.

	▲	reactormonk 3 hours ago \| parent [-]
		And some hijacking of the old DotA forums to advertise LoL: https://news.ycombinator.com/item?id=13439036