This is a bit different. It means that “Amonbofis (your competitor) is resentful toward you”. En vouloir à quelqu’un means to be resentful.

“Il t’en veut” = “il te en veut” = “il en veut à toi”, he is mad at you

“t’en” = “te en” = “envers toi” = towards/at you

But we cannot say “te en”, which is why we replace it by “t’en”.

Another example is “je t’en prie” for “if it pleases you” (“you’re welcome”).

In both case it is a sentiment (negative or positive) express towards you, at you.

Je te préviens, Numérobis, ton concurrent, t’en veut beaucoup d’avoir été choisi à sa place pour construire le palais de César.

becomes

I warn you, Edifis, your competitor, is really mad at you for having been chosen in his place to build the palace of Caesar.

The selected answer is wrong.

Most of the time, when there are commas, you can take off the sentence in the middle, because they only add information.

The sentence here is a bit tricky because it goes like :

• “Je te préviens,”
• “Numérobis, …, t’en veut beaucoup d’avoir été choisi par César à sa place.”
• “, ton concurrent,”

@Frank correctly translated the expression but nobody is resentful to Numérobis, because Numérobis is the resentful concurrent.