  We demonstrate the existence of finite-system multicriticality in a
qubit-boson model where biased qubits collectively coupled to a single-mode
bosonic field. The interplay between biases and boson-qubit coupling produces a
rich phase diagram showing multiple superradiant phases and phase boundaries of
different orders. In particular, multiple phases can become indistinguishable
in appropriate bias configurations, which is the signature of multicriticality.
A series of universality classes characterizing these multicritical points are
identified. Moreover, we present a trapped-ion realization with the potential
to explore the multicritical phenomena experimentally using a small number of
ions. The results open a novel way to probe multicritical universality classes
in experiments.
