One of the major highlights of The Beginning After the End episode 5 was when Tessia's grandfather, Virion, observed Arthur Leywin's chest pain and realized that the boy had inherited a Beast Will. The former King of Elenoir further implied that Arthur would perish if he didn't undergo training to harness his hidden power.
As fans may remember, Sylvia, the Dragon, imbued something into Arthur, aside from handing him a feather and an egg-like object, before she parted ways with him. Since then, Arthur has experienced sharp pain in his chest. The latest episode revealed that the pain Arthur experienced was connected to his inheriting the "Beast Will." So, the question is, what is a Beast Will?
According to TurtleMe's The Beginning After the End webnovel and webtoon series, a Beast Will refers to the "Will," or rather, an ability, that a Mana Beast of A-class or higher passes to a person. Usually, Mana Beasts pass their will onto their offspring. However, in rare cases, humans, elves, and dwarves can obtain the will by acquiring their Mana Core and become Beast Tamers.
Disclaimer: This article contains spoilers from The Beginning After the End webnovel/webtoon.
Exploring the concept of Beast Will in The Beginning After the End
As explained, a Beast Will is the will or inherent ability of a Mana Beast (A class or above) that is passed on to a person or, in rare circumstances, acquired by them. In The Beginning After the End, Sylvia granted Arthur her Beast Will before passing away. Normally, a Mana Beast passes the will on to their offspring, but in Arthur and Sylvia's case, the Dragon perceived Arthur as its own.
She adored the boy so much that she left him with a hidden power to bolster his strength for the future. Now, acquiring a Beast Will isn't exclusive to Mana Beasts or Humans. The Beginning After the End webnovel and webtoon reveal that Virion, Tessia's grandfather, has also inherited a Beast Will despite being an elf. Those who harness the powers of a Beast Will become "Beast Tamers."

There's another way to acquire a Beast Will besides inheriting it directly from a Mana Beast. In rare circumstances, a Mana Beast may drop its Mana Core after death, allowing one to obtain their Beast Will from that Mana Core. However, this is uncommon since most Mana Beasts destroy their Beast Will when they die.
It's also important to know that proper training is required to harness Beast Will. Otherwise, the individual may experience pain in their body, much like Arthur Leywin. In the worst scenario, the inheritor could die. That's why Arthur trained under Virion to become adept at using his Beast Will in The Beginning After the End.

Furthermore, harnessing the Beast Will has stages. According to The Beginning After the End webnovel, there are two main stages to utilizing the Beast Will's abilities. There is also a third stage that Arthur awakens to later in the novels, which will be discussed soon.
The first stage is known as the Acquire Phase. It is the initial stage where a Beast Tamer (the inheritor of the Beast Will) borrows strength from the Will. Some Beast Tamers can acquire forms similar to the Mana Beasts from which they received the Will.
The second stage is called the Integrate Stage, where a Beast Tamer fully utilizes the Mana Beast's abilities and traits. In this state, the Tamer's body transforms into that of the Mana Beast in question.
Does Arthur become a Beast Tamer in The Beginning After the End?
The anime adaptation has only revealed Arthur as the inheritor of a Beast Will thus far. However, throughout the narrative, the boy masters Sylvia's Beast Will. According to The Beginning After the End webnovel and webtoon, Arthur trained under Virion to utilize the true potential of Sylvia's powers. While it wasn't easy, the boy harnessed the strength through sheer dedication and became a Beast Tamer.
As a Beast Tamer, Arthur Leywin acquired some impressive abilities, using the powers of the Acquire Phase, Integrate Phase, and a hidden Third Phase. This Third Phase acts like an advanced form of the Integrate Phase.
According to The Beginning After the End series, Arthur's Acquire Phase allows him to demonstrate Sylvia's Static Void. In this phase, Arthur Leywin uses Aether around him to freeze time. Yet, it doesn't include those who can utilize Aether in the same way.
Notably, it is not an ideal moveset for Arthur, as he cannot interact with frozen people or objects. Most importantly, the mana particles in the atmosphere also come to a standstill in this phase.

At the same time, it used to take a lot of toll on Arthur's body when he initially learned to demonstrate it. Later in the series, Arthur learned the Integrate Phase of Sylvia's Beast Will. In this phase, Arthur can enter the Realmheart form, resembling the dragon, Sylvia.
Arthur's Realmheart phase transforms his appearance; his hair becomes long and white, his eye color changes to lavender, and golden runes cover his arms and under his eyes. As a result, Arthur can see all the Mana and Aether surrounding him and utilize them effectively. For instance, he can use Gravity Magic in the Realmheart Form, which he cannot do otherwise.
Additionally, Arthur gained the ability to cancel spells with this form after training with Myre. Through intense training, Arthur awakened a Third Phase, which is considered the advanced level of the Second Phase. In the third phase, Arthur Leywin can unleash the true extent of the Realmheart form and control the Spatium aspect of Aether.
Thus, he can utilize the premature form of Godstep, enabling him to create short-distance portals. Furthermore, his physical attacks are imbued with aether. Therefore, Arthur's inner strength is significantly boosted by this Advanced Stage.
Conclusion

The Beast Will is one of the complex power systems that TurtleMe has introduced to enrich The Beginning After the End series. Arthur Leywin eventually masters Sylvia's Will and becomes a true Beast Tamer. It's exhilarating how one can harness a Mana Beast's powers in this series after becoming attuned to their Will.
Also read:
- The Beginning After the End episode 5 full highlights
- The Beginning After the End anime's "slideshow" animation has fans convinced it's worse than Blue Lock season 2
- 10 anime to watch if you like The Beginning After the End