Alatar does not get assigned because there is no assignment phase once he becomes a minion. He is fierce so that he can fight once in the turn that he appears (in the fierce fight because after the skirmish phase when he becomes a minion there is a fierce assignment phase when he must then be assigned).
For clarification, the turn order happens like this:
• Assignment phase - when minions are assigned
• Skirmish phases - when
Alatar Deceived becomes a minion (he was not a minion during the assignment phase and therefore was not assigned)
• Fierce Assignment phase - when fierce minions are assigned, including
Alatar• Fierce skirmish phases - when
Alatar and other fierce minions fight
Thranduil