I'm not sure if it's really the Chitauri mastermind, but bear with me.
In the opening of The Avengers we see some blue glimmer, some Chitauri, Loki's hands receiving the scepter/spear. All overlaid by these lines
He (meaning Loki) is ready to lead
And our force, our Chitauri, will follow
A world will be his
The universe, yours.
Who is this "you", the omniscient voice is talking to? Who did the Chitauri strike a deal with?