Official Riftbound ruling

Does Svellsongur copy text that the unit has gained from other sources, such as the Assault granted by Cleave? What about text it’s gained from other Equipment?

No, Svellsongur only copies the text that’s actually printed on the unit. This is an ambiguity in the current rules, which refer to “appending” and “removing” rules text without reference to whether that text is original to the unit or not.

The key word here is actually copy, a system that will be fleshed out in a future rules update. For now, we’ll just tell you that copies of objects or text take on their original traits, without regard to any subsequent alterations.

Svellsongur doesn’t grant the unit additional “copies” of the abilities of other Equipment. So putting 3 Svellsongur on the same unit will give that unit four total instances of its abilities, not eight. (Four should be plenty.)

Riot's question and answer, word for word, from theRiftbound Spiritforged FAQ

The cards

  • Svellsongur
    Svellsongur

    Gear

    Equip 1 Energy1 EnergyCalm RuneCalm Rune (1 Energy1 EnergyCalm RuneCalm Rune: Attach this to a unit you control.)

    As this is attached to a unit, copy that unit's text to this Equipment's effect text for as long as this is attached to it.

  • Cleave
    Cleave

    Spell

    Action (Play on your turn or in showdowns.)

    Give a unit Assault 3 this turn. (+3 MightMight while it's an attacker.)