In Leviticus 18:21 in the LXX it reads..
"And thou shalt not give of thy seed to serve a ruler; and thou shalt not profane my holy name; I the Lord"
... which is different from the Masoretic rendering of Molech, which reads:
You shall not give any of your children to offer them to Molech, and so profane the name of your God: I am the LORD.
The sudden mention of Molech in this passage on fornication in the Masoretic has always stood out to me as odd, especially when it is addressed later in Lev 20. (incidentally, the LXX does go on to mention Molech specifically in Lev 20.)
So I'm thinking the LXX may be closer to the original in this verse. But can anyone shed more light on the Greek "To serve a ruler"? It seems pretty vague.