The description says a creature can only take the damage once per round (unless it ends its movement in the arc of its own accord and therefore takes the damage again. Meaning they need to move if in the arc).

Can Lugh (let's call him creature #1), or the other person in the arc (creature #2) move while this is in effect? Given that it says within 30 feet I would assume yet, but that it's broken if they move beyond the 30 foot limit.

If creature #1 moved such that the arc would pass across another, third, creature, would that trigger the third creature having to make a save? I would say yes. But what if creature #2 then used their move to cause the arc to once again pass over that third creature? Given that each turn is supposed to represent only a few seconds, I would say no.

Ring of the Storm Lizard.

Silver ring with picture of stormlizard in blue gem. Maximum of 6 charges (regains 1d6 at dawn).
Use an action to:
1. Expend 1-3 charges and speak command work 'Charge' (in Draconic) to attack one creature you can see within 60 feet of you. The ring produces a spectral stormlizard and makes an attack with a +7 bonus. On a hit, for each charge you spend, the target takes 2d10 force damage and is pushed 5 feet away from you.
2. Expend 4 charges & Draconic command word 'Bolt' to cast lightning bolt DC 15.
3. Expend 1 charge and say Draconic command word 'Arc' to form a lightning arc between you and another creature within 30 feet. If the target is unwilling, then they can make a DC 15 Wisdom save and the charge is wasted and nothing happens. If the arc is complete, you and the target gain immunity to lightning damage whilst the arc remains. The arc lasts until the start of your next turn but can be continued by an extra round by expending another charge. The first time each round that a creature passes through the line joining you and the target takes 3d6 lightning damage, half on a DC 15 Dexterity saving throw. Any creature that ends its turn in this line takes the damage again and does not get a saving throw.
Requires attunement.
History: (DC 10) One of the many items made for the followers of Tiamat.

Just getting this post ready to reveal what you know when Lugh attunes to it. Remember, you can only attune to 3 items. This would be Lugh's 3rd (inc the two above) unless you have another I have not listed?

But yeah, I would say it is b/c of the attunement :)

Also, you are sure this is a significant relic of the past. You just don't know anything relating directly to it (unlike your knowledge of the lute you carry - you have a feeling this staff's story is just as interesting…

1. +1 AC. 10 charges (regains 1d6 + 4 at dawn; if all expended shatters on a 1 on d20 roll). (If on spell list) can cast: mage armor (1 charge) or shield (2 charges).
2. Requires attunement.
3. Minor Property: Resuscitator - wielder gets +2 on first death save made each day.

1. All spells used to charm = saves vs are made at disadvantage if lute is used as arcane focus. An action to play & cast one spell from: animal friendship, protection from energy (fire only), protection from poison. Cannot cast that spell again until next dawn. Spells use caster's spellcasting ability. Requires attunement.
2. Quirk: Try to play when not attuned DC 15 Wis save or take 2d4 psychic dam.
3. Signature Trait: When you hold the lute it is calming. Music typically played is soothing and relaxing to the audience.
4. Minor Property: Hardy. If the attuned creature has the lute with them during a long rest, they regain 1 extra hit die in the morning.
5. Origin: (DC 18) Half-elven. This is the lute of Doss, who established the Doss College. He was famed for protecting the elves and rural folk from the dragonborn and at one stage a dragon.
6. History: (DC 15) Doss College was located in the forests of central Nerath and its primary focus was nature-magic. (DC 15) Doss was one of the ancient Bael Nerath bardic colleges that were key in the underground resistance of dragonborn rule. These colleges were also the nexus of the merging of human and elven culture in the region.

BTW: Leather armor = light (as it is light armor).