Wait for the counter to finish to get Wriggling Tentacle Sashimi.
Only 1 is necessary to complete the Adventure's Tome, after that, you can sell/destroy it.

it took me 3 attempts to get the tentacle

20 kills - no drop

Took me 7 kills to get

Took me 2 kills to get it

Mata a Rovlen para conseguir el tentáculo, no siempre lo tira. 5 veces lo he tenido que matar. Debes esperar 30 minutos para usarlo.
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Kill Rovlen to get it, it has medium drop rate so try to kill some times if you dont get it at first time. I had to killed it 5 times.
You must wait 30 minutes to use it.

Can we loot him only on the first daily attempt ? EDIT : NO

EDIT : confirmed at the moment : we can loot it at the third kill of the day.
Unfortunately, it wasn't me :'(

No one else said this here, but you can also kill him several time's in a row if he is up in multiple instances. On my server at least, there's usually 3 to 4 instances of Bilbrin Forest active during the day, so I was consistently able to kill him 3 to 4 times every time I went there just by changing the "channel" that I was in.

Took me 3 visits of killing him 3 to 4 times each time to get the drop.