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Artist's commentary
「Mmm... Be welcome inside me」
「Please enjoy all the pleasures it offers」
I never thought that the final boss that shocked the EXTRA series fans would turn into a servant... I'm so happy to welcome you...
The words for servant (サーバント, "saabanto") and mackerel (鯖, "saba") have a similar sound in Japanese and are often used interchangeably. In this post it's literally written "would turn into a mackerel".
Though it wasn't an intended pun, Kiara got a summer version with fish traits in FGO years after this illustration was made.