What could turn out to be an ugly court battle has commenced between developer Valve and Activision Blizzard, with the former filing suit against the publisher on Tuesday.

Valve alleges that Activision Blizzard underpaid it almost half a million dollars in royalty dues owed from an earlier dispute between the developer and Sierra, whose assets and contractual duties were taken on by Activision when it merged with Sierra parent Vivendi.

According to Valve, both parties agreed earlier this month on an arbitrator's decision awarding the developer $2,391,932, reports GamePolitics, which unearthed a report documenting the process.

Activision then challenged the award, claiming that Valve had previously been overpaid by $424,136. While Valve responded by saying that Activision hadn’t raised this during the arbitration process, the publisher wrote Valve a cheque last week for $1,967,796 - the amount awarded by the arbitrator minus the $424K.

Valve has since taken the matter to a US District Court in Seattle in a bid to get back the rest of the money it believes it is owed, and said in its court filing that Activision is ready to launch a counter-suit against the claim.