so, i was wondering few months ago about this, but just today i did some research
1. video) ~2 months ago, when i was watching GK pvp tournament, i was wondering why they have never used mass cubic there (every party had 1-4 summoners), but i thought maybe its OP and its forbidden by rules that u cannot use them
2. video) then i got link with stream of Silent Team cp (hero aw, was curious about his gameplay), this CP has in setup phantom summoner, and last some streams were kind of PvE oriented in GC and when i saw they are not using mass phantom cubic even on pve there, i realized it might not exist?
so i went to check l2wiki and l2central, and i couldnt find the spellbook for mass phantom cubic, and when i searched in phantom summoner skills, it didnt exists either
(if it was added as custom thing on this server, then i had to miss it, but i am sure that only custom thing on summoners are the buff pets)