Spells and Old magic

Alathar gazed upon the mighty stone fortress from his vantage point upon a nearby hill, the countless banners of his army snapping in the wind. Upon his gesture the elven priests began chanting, casting sacred oils and salts before them as they approached him. The second priest opened the ornate chest in his arms, presenting it to Alathar.
Showing nothing but stalwart determination on the outside the elven commander gently reached into the chest, giddily grasping an item covered in a smooth silk cloth, scented with the smell of flowers and embroidered with scenes of war, myths and power. Finally unfolding the cloth, Alathar could not help but squirm as a mix of emotions rushed through him. There it lay, a crystal in the colour of thought. Raw power in his hands.
Grasping it in his hands, Alathar began reciting the old texts as he focused all thought on the stone bastion before him. As he finished, a blinding light sourrounded the fortress, followerd by a sharp and loud crack. Alathar grinned as the crystal dissolved in his hands and the troops cheered, the castle reduced to rubble.

Effect

Old magic is the most common form of magic across the continent, most often used for the purposes of warfare and destruction, but it also has a variety of other uses. Functionally Old magic can do nearly anything provided one has a large enough amount: One can almost any knowledge, save perhaps for those more philosophical questions (asking to know the meaning of life for example induces no reaction) or the knowledge of others (the effects of which are discussed below). Put simply, Old Magic is pure energy in crystalline form prepared to be harnessed in a multitude of ways. It can create and destroy, but not conceal or shield in any meaningful capacity.
Perhaps the most common use of Old Magic is that of combat, utilized as it is by countless warriors across the continent to perform feats large and small. It is famously one of few things able to pierce a dragons scales and has been utilized by a many a mythical hero to bring dragons of equal renown to their knees, earning it the simple name "death" in their tounge.

Side/Secondary Effects

The first and most important rule any novice learns when initiated into the art of handling Old Magic is this: Any interaction between soul magic and Old Magic will inevitably be met with disaster. This has been the downfall of many an master of intrigue and simple knaves unaware of the complexities of old magic, who are found motionless with eyes as dark as coal after foolishly trying to gaze into the mind of someone else. What exactly causes this is unknown, but like oil and water, the two types of magic do not get along, with devastating effects if someone forces them together.
Other than its interactions with Sould Magic, there is not much in the area of side effect to take into consideration, save perhaps any effects you may have desired in the first place. Many are alarmed that the crystal withers away after use and believe they have performed the spell in an erroneous fashion, but this is simply what happens after spending the magic when casting.

Manifestation

Old Magic manifests itself in an innumerable number of beautiful ways, most often dependent on the desired outcome. It is often accompanied by light or heat as magic energy is transferred or consumed, but if it involves simpler outcomes like bringing rain, nothing beyond the desired outcome is manifested.

Source

The most plentiful sources of Old Magic in the world are also often the most remote and dangerous, nearly always confined to areas like mountains ranges and winding caves.There are a number of theories to explain this remoteness. Some say it is simply because all the sources of old magic easy to access have been depleted, whilst others think the vast amounts of soul magic in inhabited lowland areas "repel" Old magic and force it into the mountains were sould magic is near nonexistent. It is not currently known how plentiful Old Magic is, if it remanifests after consumption or if it is created in some manner similar to substances like Gold and Iron.
This makes Old Magic perhaps the most valuable ware on the continent and a large pebble of Old Magic can cost as much as an entire well-off farmstead plus a years worth of pay, while a palm-sized piece may get you a domesticated Elephant. Thus the extraction and trade of Old Magic is highly taxed and restricted in those areas controlled by any larger realm and only approved adventurers may even attempt to venture into these areas to try their luck, leading to the rise of many prominent adventuring guilds whose sole mission is to engage in the aquisition and protection of Old Magic and its trade routes.

Discovery

Old Magic has been around since the dawn of time and countless beings have utilized it in countless ways.

Gestures & Ritual

Rituals, spells, traditons and mantras are all common ways of practicing old magic found across the world and are all in the end equally useless. The only things needed to channel old magic is skin contact and a focused thought or wish. One can practice Old magic with only the crystal and no rituals, but not the other way around.
In older, simpler times, it was believed that the rituals and chants were involved were of vital importance when channeling old magic, ingraining many of the old traditions firmly into spellcasting across the world, generating a wealth of different styles for spellcasting.
Archaic speech is a common part of these rituals and a useful channeling tool as it often tends to be more long-winded and descriptive, helping the caster focus more fully on the desired effect and also minimizing the risk of stray thoughts or ambiguous wording altering the spell.