Gráinne, daughter of king Cormac mac Airt, princess of Tara, and bride-to-be, was furious.

Fionn Mac Cumhail, leader of the Fianna, Ireland's most celebrated band of warriors, had just arrived at her castle.

Gráinne had believed she was betrothed to a young soldier, bold and brilliant enough to be her equal.

Yet, upon their arrival, it became clear she was expected to marry Fionn himself, who, while once a legend, was far past his prime.

Gráinne would not suffer this insult.

Before the wedding, the King held a great banquet.

As the warriors told tales of Fionn's strength and magical healing hands, Gráinne waited to enact her revenge.

But while scanning the room, her eyes locked on to one of the guests.

Diarmuid, one of Fionn's bravest soldiers and foster child of the God of Love himself, was renowned for his stunning beauty.

Perhaps this was the bold warrior the princess had imagined.

Only one way to find out.

Gráinne swept into action, slipping a sleeping draught into the partygoers' goblets.

Soon, the hall fell silent, save for two unenchanted guests.

The pair immediately felt a powerful connection, but Diarmuid hesitated.

He'd never put his interests before the Fiannas'.

Seeing Diarmuid waver between duty and desire, Gráinne took the decision out of his hands.

With a light kiss, she placed a geis on his brow, an ancient spell that bound them together, for better or worse.

As the pair escaped, Diarmuid's heart was racing.

Had he really betrayed Fionn for a woman he'd just met?

To answer his son's questions, Aengus Óg appeared beside them.

The God of Love blessed their union and eased his son's concern.

But he also warned that endless difficulties lay ahead.

At the palace, Fionn awoke, full of wrath.

He rallied an epic war band that quickly caught up to the lovers.

To buy time, Diarmuid and Gráinne conjured a massive enclosure with seven doors.

While Gráinne escaped with Angus Óg, Diarmuid went door to door, seeking Fionn.

Some of the Fianna sought peaceful surrender, others itched for combat.

Diarmuid gave neither.

But when he finally heard his leader's voice calling for blood, he knew there was no turning back.

Launching himself over the barricade, Diarmuid leapt to Gráinne's side.

The chase was on.

The pair ventured west, where they soon encountered the giant Modan.

Eager to help the lovers, he guarded them at night and carried them by day.

And once the pair had crossed countless rivers and hills, their fear began to ease.

They left the forest's cover, laughing, flirting, and growing bolder each day.

But Fionn saw their contentment was an opportunity.

He enlisted three sea chiefs to capture the wandering lovers.

Diarmuid scared them off with mystifying sword tricks, only for Fionn to send poisonous hounds.

Diarmuid and Gráinne fled, yet again, into an even more tangled forest, a sight which filled the Fianna with exhaustion.

They implored Fionn to give up the hunt, and at this, their leader grew quiet.

He'd already lost a dear friend; now, it seemed his wrath had cost him the respect of his men as well.

Swallowing his rage, he called off the chase.

For years, Gráinne and Diarmuid lived in peace.

In time, Angus Óg even brokered an agreement between the warriors, and their conflict faded into memory.

But Gráinne never forgot the Love God's warning.

One day, Fionn invited Diarmuid on a boar hunt high in the mountains.

While the two were riding, a venomous boar charged them and mortally wounded Diarmuid.

As he bled, the old warrior begged his friend for water, as all the Fianna knew, drinking from the hands of Fionn Mac Cumhail could save their lives.

But as Fionn turned to aid Diarmuid, his old resentments came rushing back.

For just a moment, his fingers parted, letting the water trickle to the ground.

And by the time Fionn realized his mistake, it was too late.

And the embers of rage that long burned inside him were finally quenched by tears.

Follow Fionn's son Oisín on his journey to the Otherworld, a land where time stands still and people never age, and discover the simple mistake that doomed him forever with this myth