Bowser is in another castle?
(Teki wa Honnouji ni ari; “The enemy is in Honnou Temple”)
One’s true purpose is “elsewhere.” Someone’s true goals are not what they were stated to be – not because they were confused about what they wanted, but because they were lying to gain a tactical advantage.
This complete sentence begins with the noun 敵 (teki), “enemy,” emphasized by the particle は (wa). Next we have the proper noun 本能寺 (Honnouji), a Buddhist temple in Kyoto that has stood since 1415… and which saw a pivotal event in the wars of the Sengoku period. Anyway, this place name is marked by the location particle に (ni), and the whole is capped by the verb あり (ari), “to be,” in sentence-final form.
The Honnouji Incident is a famous betrayal, referenced time and time again in Japanese culture. Oda Nobunaga, one of the major warlords of the time, sent a general named Akechi Mitsuhide to help prosecute a siege on a castle. But Akechi marched his forces by Kyoto, where Oda was stopping over in the temple. For reasons that are still a matter of speculation, he is said to have betrayed his master by declaring “The enemy is in Honnou Temple” and ordering his troops to attack. Oda eventually set fire to the rooms he was staying in and committed seppuku. You can see a little dramatic animation describing the incident in this Extra History video, part of a multi-video series about the Sengoku period.
It is acceptable to write the final verb with the kanji 在り.
The saying also exists in yojijukugo form, as 敵本主義 (teki-hon-shugi), meaning a policy of hiding one’s motives from one’s enemies until the last moment.
(“Kono ko, sugoi na. Kouen de asobou to itteta kuse ni, teki wa Honnouji ni ari de, hontou no nerai wa aisu wo katte morau koto datta.”)
[“This kid is amazing. He was all, ‘Let’s go play in the park!,’ but that was just a bit of misdirection – his real goal was to get me to buy him some ice cream.”]