Disclaimer : this is an ultra-specific oddball that will likely never happen to anyone but me. Read only if you have time to kill !
Shapechange is a 9th level spell that allows a player character to assume the form of another creature, while retaining their race/class/other benefits if the new form can physically use them. While the spell is normally restricted to Self-only, it is allegedly possible to make a spell glyph of this spell (requiring access to two 9th level spell slots, such as with a Tome of the Stilled Tongue or a Boon of High Magic) to activate onto a different target than the glyph caster — opinions vary on whether bypassing a self-requirement in this way is possible, but for the purpose of this question, let’s assume that it works.
The Frost Giant Everlasting One has an ability called Vaprak’s Rage, which is similar to, but not identical to a Barbarian’s Rage class feature :
As a bonus action, the giant can enter a rage at the start of its turn. The rage lasts for 1 minute or until the giant is incapacitated. While raging, the giant gains the following benefits: - The giant has advantage on Strength checks and Strength saving throws - When it makes a melee weapon attack, the giant gains a +4 bonus to the damage roll. - The giant has resistance to bludgeoning, piercing, and slashing damage.
Suppose that a Wizard makes a spell glyph of shapechange, and then, that a Barbarian triggers the glyph and assumes the form of a Frost Giant Everlasting One.
Now that our premise has been established (a character with access to Rage and Vaprak’s Rage at the same time), several questions emerge :
Can the Shapechanged Barbarian benefit from both Rage and Vaprak’s Rage, gaining both damage increases ? (Not sure whether the “Same Name Effects Stacking” rule applies here)
If, for some reason, the Barbarian has access to spellcasting, can he cast spells & concentrate on them while Vaprak-Raging but not regular-raging ?
Are his Rage-related class features, such as Frenzy (if the Barbarian is a Berserker) and Relentless Rage, “enabled” while Vaprak-Raging but not regular-raging ?
Other interactions I may not have thought about, etc.