Blizzard is the biggest player in online gaming and Warcraft is the global market leader of what are known as massively multi-player online role-playing games, or MMORPGs.
It is currently owned by the French media group Vivendi.
As part of the merger plan, Blizzard will invest $2bn in the new company, while Activision is putting up $1bn.
The merged business will be called Activision Blizzard and its chief executive will be Activision's current CEO Bobby Kotick. Vivendi will be the biggest shareholder in the group.