As a experienced DM I think staff weapon will be two-handed weapon, it is too long for one hand. It might come with reach and 1d10+STR bludgeoning damage or 2d6+STR/1d12+STR without reach. It looks pretty heavy.
It is alien ranged beam weapon so most of the players won't be profficient with it but still can have a lucky shot. Problem is players or NPC on higher lvl will hit almost all the time, so disadvantige on attacks or flat -5 for hit for example might be good idea.
For damage I think it will be force because that is the most dangerous type of damage that is similar to beam weap