Erik’s first coup against T’Challa was successful, but the Black Panther later bounced back to defeat his enemy, and during their battle (but not by Black Panther’s hand), Erik was killed.
In traditional comic book fashion, he was later resurrected, and since then hasn’t let up in his attempts to control Wakanda.
Once there, Erik began collecting resources and allies to help him seize the Wakandan throne and take the country.
Pretending to be a Wakandan who simply wanted his exile from Wakanda to end, Erik Killmonger made contact with T’Challa in New York and secured repatriation back to the African nation.