  The functional determinant is computed exactly for quantum oscillations about
periodic instantons with non-trivial values of the Polyakov line at spatial
infinity (or holonomy). Such instantons can be viewed as composed of the
Bogomolnyi--Prasad--Sommerfeld (BPS) monopoles or dyons. We find the weight or
the probability with which dyons occur in the pure Yang--Mills partition
function. It turns out that dyons experience quantum interactions having the
familiar "linear plus Coulomb" form but with the "string tension'' depending on
the holonomy. We present an argument that at temperatures below the critical
one computed from Lambda_QCD, trivial holonomy becomes unstable, with
instantons ``ionizing'' into separate dyons. It may serve as a microscopic
mechanism of the confinement-deconfinement phase transition.
