Reaction of antimony, selenium, and selenium(IV) chloride in the Lewis acidic ionic liquid [BMIM]Cl/AlCl₃ (BMIM: 1-n-butyl-3- methylimidazolium) at room temperature yielded air-sensitive black block-shaped crystals of [Sb₁₀Se₁₀][AlCl₄]₂. The triclinic unit cell (space group ${P\bar 1}$, a=947.85(2), b=957.79(2), c=1166.31(3) pm; α=103.622(1), β=110.318(1), γ=99.868(1)°; Z=1) contains the first mixed antimony/selenium polycation, [Sb₁₀Se₁₀]²⁺. The centrosymmetric polycyclic cation consists of two realgar-like [Sb₄Se₄] cages, which are connected through positively charged, three-bonded selenium atoms with a central [Sb₂Se₂] ring. Quantum chemical calculations predict semiconducting behavior of the compound and indicate primarily covalent bonding with varying ionic contribution within the [Sb ₁₀Se₁₀]²⁺ polycation, while the interactions between the polycation and the [AlCl₄]^- anions are predominantly ionic. The applicability of the Zintl concept to the chemical bonding in the heteronuclear polycation was evaluated by a thorough quantum chemical analysis.