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TL;DR: Arcana Unearthed_/_Arcana Evolved by Monte Cook (Evolved being an updated version of Unearthed). Though they are reference books for his Dungeons & Dragons setting rather than a novel. There were two short-story collections in the setting: Children of the Rune and The Dragons' Return.

From the Wikipedia:

...Chi-Julud, in which the wise giants temporarily lose their wisdom to become stronger and more warlike.

Looking at the question again, it occurred to me that this sounded like the Aiel War in Robert Jordan's Wheel of Time series rather than anything from A Song of Ice and Fire. So, I looked on Worldbuilding and the Aiel War only is discussed once in this answer.

Here is the TV Tropes quote the answer includes:

When King Laman of Cairhien cut down the Avendoraldera, the tree that the mysterious desertdwelling Aiel-people had given his people as an sign of friendship hundreds of year earlier in order to carve himself a throne, four of the twelve Aiel Clans crossed the Dragonwall and invaded the Westlands. For the Westlands It was the greatest war they had seen in centuries, uniting almost every country under one banner that was stil unable to stop the Aiel horde. To the Aiel it was simply the execution of the Oathbreaker King Laman, and once they succeded in that they turned around, returning to their desert without giving any of the lands they had conqured a second glance.

Jerenda goes on to describe Arcana Evolved and it's giants (emphasis mine):

For an entirely different view, I am stealing madly from Monte Cook's Arcana Evolved. In his book, the giants treat war very seriously, to the point where they have a ritual that they undergo before they engage in warfare. This ritual literally changes their body, making them more ferocious and more likely to win.

TL;DR: Arcana Unearthed_/_Arcana Evolved by Monte Cook (Evolved being an updated version of Unearthed). Though they are reference books for his Dungeons & Dragons setting rather than a novel. There were two short-story collections in the setting: Children of the Rune and The Dragons' Return.

From the Wikipedia:

...Chi-Julud, in which the wise giants temporarily lose their wisdom to become stronger and more warlike.

Looking at the question again, it occurred to me that this sounded like the Aiel War in Robert Jordan's Wheel of Time series rather than anything from A Song of Ice and Fire. So, I looked on Worldbuilding and the Aiel War only is discussed once in this answer.

Here is the TV Tropes quote the answer includes:

When King Laman of Cairhien cut down the Avendoraldera, the tree that the mysterious desertdwelling Aiel-people had given his people as an sign of friendship hundreds of year earlier in order to carve himself a throne, four of the twelve Aiel Clans crossed the Dragonwall and invaded the Westlands. For the Westlands It was the greatest war they had seen in centuries, uniting almost every country under one banner that was stil unable to stop the Aiel horde. To the Aiel it was simply the execution of the Oathbreaker King Laman, and once they succeded in that they turned around, returning to their desert without giving any of the lands they had conqured a second glance.

Jerenda goes on to describe Arcana Evolved and its giants (emphasis mine):

For an entirely different view, I am stealing madly from Monte Cook's Arcana Evolved. In his book, the giants treat war very seriously, to the point where they have a ritual that they undergo before they engage in warfare. This ritual literally changes their body, making them more ferocious and more likely to win.

TL;DR: Arcana Unearthed/Arcana Evolved by Monte Cook (not sure I understand the distinction). Though they are reference books for his Dungeons & Dragons setting rather than a novel. There are definitely novel tie-ins to the setting that I will track down.

From the Wikipedia:

...Chi-Julud, in which the wise giants temporarily lose their wisdom to become stronger and more warlike.

Looking at the question again, it occurred to me that this sounded like the Aiel War in Robert Jordan's Wheel of Time series rather than anything from A Song of Ice and Fire. So, I looked on Worldbuilding and the Aiel War only is discussed once in this answer.

Here is the TV Tropes quote the answer includes:

When King Laman of Cairhien cut down the Avendoraldera, the tree that the mysterious desertdwelling Aiel-people had given his people as an sign of friendship hundreds of year earlier in order to carve himself a throne, four of the twelve Aiel Clans crossed the Dragonwall and invaded the Westlands. For the Westlands It was the greatest war they had seen in centuries, uniting almost every country under one banner that was stil unable to stop the Aiel horde. To the Aiel it was simply the execution of the Oathbreaker King Laman, and once they succeded in that they turned around, returning to their desert without giving any of the lands they had conqured a second glance.

Jerenda goes on to describe Arcana Evolved and it's giants (emphasis mine):

For an entirely different view, I am stealing madly from Monte Cook's Arcana Evolved. In his book, the giants treat war very seriously, to the point where they have a ritual that they undergo before they engage in warfare. This ritual literally changes their body, making them more ferocious and more likely to win.

TL;DR: Arcana Unearthed_/_Arcana Evolved by Monte Cook (Evolved being an updated version of Unearthed). Though they are reference books for his Dungeons & Dragons setting rather than a novel. There were two short-story collections in the setting: Children of the Rune and The Dragons' Return.

From the Wikipedia:

...Chi-Julud, in which the wise giants temporarily lose their wisdom to become stronger and more warlike.

Looking at the question again, it occurred to me that this sounded like the Aiel War in Robert Jordan's Wheel of Time series rather than anything from A Song of Ice and Fire. So, I looked on Worldbuilding and the Aiel War only is discussed once in this answer.

Here is the TV Tropes quote the answer includes:

When King Laman of Cairhien cut down the Avendoraldera, the tree that the mysterious desertdwelling Aiel-people had given his people as an sign of friendship hundreds of year earlier in order to carve himself a throne, four of the twelve Aiel Clans crossed the Dragonwall and invaded the Westlands. For the Westlands It was the greatest war they had seen in centuries, uniting almost every country under one banner that was stil unable to stop the Aiel horde. To the Aiel it was simply the execution of the Oathbreaker King Laman, and once they succeded in that they turned around, returning to their desert without giving any of the lands they had conqured a second glance.

Jerenda goes on to describe Arcana Evolved and it's giants (emphasis mine):

For an entirely different view, I am stealing madly from Monte Cook's Arcana Evolved. In his book, the giants treat war very seriously, to the point where they have a ritual that they undergo before they engage in warfare. This ritual literally changes their body, making them more ferocious and more likely to win.

TL;DR: Arcana Evolved by Monte Cook. Though it is a reference book for his Dungeons & Dragons setting rather than a novel.

From the Wikipedia:

...Chi-Julud, in which the wise giants temporarily lose their wisdom to become stronger and more warlike.

Looking at the question again, it occurred to me that this sounded like the Aiel War in Robert Jordan's Wheel of Time series rather than anything from A Song of Ice and Fire. So, I looked on Worldbuilding and the Aiel War only is discussed once in this answer.

Here is the TV Tropes quote the answer includes:

When King Laman of Cairhien cut down the Avendoraldera, the tree that the mysterious desertdwelling Aiel-people had given his people as an sign of friendship hundreds of year earlier in order to carve himself a throne, four of the twelve Aiel Clans crossed the Dragonwall and invaded the Westlands. For the Westlands It was the greatest war they had seen in centuries, uniting almost every country under one banner that was stil unable to stop the Aiel horde. To the Aiel it was simply the execution of the Oathbreaker King Laman, and once they succeded in that they turned around, returning to their desert without giving any of the lands they had conqured a second glance.

Jerenda goes on to describe Arcana Evolved and it's giants (emphasis mine):

For an entirely different view, I am stealing madly from Monte Cook's Arcana Evolved. In his book, the giants treat war very seriously, to the point where they have a ritual that they undergo before they engage in warfare. This ritual literally changes their body, making them more ferocious and more likely to win.

TL;DR: Arcana Unearthed/Arcana Evolved by Monte Cook (not sure I understand the distinction). Though they are reference books for his Dungeons & Dragons setting rather than a novel. There are definitely novel tie-ins to the setting that I will track down.

From the Wikipedia:

...Chi-Julud, in which the wise giants temporarily lose their wisdom to become stronger and more warlike.

Looking at the question again, it occurred to me that this sounded like the Aiel War in Robert Jordan's Wheel of Time series rather than anything from A Song of Ice and Fire. So, I looked on Worldbuilding and the Aiel War only is discussed once in this answer.

Here is the TV Tropes quote the answer includes:

When King Laman of Cairhien cut down the Avendoraldera, the tree that the mysterious desertdwelling Aiel-people had given his people as an sign of friendship hundreds of year earlier in order to carve himself a throne, four of the twelve Aiel Clans crossed the Dragonwall and invaded the Westlands. For the Westlands It was the greatest war they had seen in centuries, uniting almost every country under one banner that was stil unable to stop the Aiel horde. To the Aiel it was simply the execution of the Oathbreaker King Laman, and once they succeded in that they turned around, returning to their desert without giving any of the lands they had conqured a second glance.

Jerenda goes on to describe Arcana Evolved and it's giants (emphasis mine):

For an entirely different view, I am stealing madly from Monte Cook's Arcana Evolved. In his book, the giants treat war very seriously, to the point where they have a ritual that they undergo before they engage in warfare. This ritual literally changes their body, making them more ferocious and more likely to win.