What caused Tanjiro to awaken his Demon Slayer Mark?

The protagonist Demon Slayer: Kimetsu no Yaiba, Tanjiro Kamado, has worked hard to earn his rightful place among the demon slayers. With rigorous training alongside Zenitsu and Inosuke and perfecting his Total Concentration, he gained an advantage while fighting the Upper Rank Six Demons in the Entertainment District.

While decapitating Gyutaro apace with the Sound Hashira, Tengen Uzui, Tanjiro awakened his Demon Slayer Mark, which becomes a game-changer by successfully separating the demon’s head from its body.

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How did Tanjiro awaken his Demon Slayer Mark?

Tanjiro was at his limits, as he could not even lift his blade in front of Gyutaro. Maintaining a frightening demeanour without raising suspicion, he managed to recuperate a little and give a powerful head-butt to the demon leaving him with a concussion. Enraged by that, the demon almost kills Tanjiro, but he gets saved by Tengen.

Fighting in conjunction with Tengen, Tanjiro awakens his Mark while beheading the demon. It is said that only those who have survived life-threatening situations with a heart rate of 200 bpm, as well as a body temperature over 39 degrees Celsius, can awaken their Demon Slayer Marks.

During the battle with Daki in episode 13, “Layered Memories” of season 2, Tanjiro went on a rampage with his Sun Breathing Technique. However, he almost had her by the neck, but he ran out of breath. This revealed that Sun Breathing requires much strength and stamina from its user.

Tanjiro has still didn’t mastered his new Breathing Technique, as he switches to Water Breathing mid-battle.

Every time he used Sun Breathing, he was seen having a hard time breathing, with his hand on his chest like he was going into a cardiac arrest. His constant pushing limits and fighting spirit triggered his Mark.

Upon appearing, Tanjiro went berserk, with his hair and powers radiating with pure blood-lust. He finally beheads Gyutaro before he can perform his Blood Demon Art.

These Marks grant their users Immense Strength and Speed during combat and dyes the user's Nichirin Blade to crimson red. With this mark, they can also use the "Transparent World" ability to see their opponents' muscles, blood flow, and joint movement with the upper hand to predict their attacks.

The user also becomes hyper-perceptive, allowing them to perceive the world around them at a much faster rate.