A System-of-Systems (SoS) is architecturally designed to exhibit emergent behavior from the interaction of independent systems. Based on the supervenience principle, an emergent behavior is a macro-scale behavior upwardly caused by a set of micro-scale behaviors according to alternative approaches: (i) endogenous, when the micro-scale behaviors originate from inside constituent systems of the SoS; (ii) exogenous, when oppositely the micro-scale behaviors originate from outside constituent systems. This paper presents the concepts and constructs of a novel Architecture Description Language (ADL), named SosADL, for exogenously describing architectural emergent behaviors of software-intensive SoSs. In particular, it demonstrates how architectural mediators expressed with SosADL supports exogenous architecture descriptions through an excerpt of a real application for architecting a Reconnaissance SoS, focusing on the flocking behavior of a fleet of Unmanned Aerial Vehicles (UAVs).