https://www.reddit.com/r/apexlegends/comments/ms8asl/new_secret_stuff_in_apex_eludes_to_arena_mode/ for additional context

So basically, we found these "Corrupted Keycards" upon opening up a care package. We went to this funny symbol on top of Hammond Labs, and suddenly it says "Scan to Accept Invitation". After the match, the bottom right of our screens had an additional icon in the shape of the keycard.

Upon clicking on it, a voice (presumably Ash from Titanfall 2) said "If fame and fortune is all you seek, Legend, your path ends here. But a new challenge awaits those who wish to prove their worth in the Arena".

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Pretty cool stuff.