What I've noticed Regulus suffers from a lot is the low HP and defense (1v1) capability especially in domination games. Here is a generic build I use on most of my Pantheon games. Speccing deep into enhanced attributes at the beginning allows for nice mana and HP regen as well as increased survivability. Sometimes I will include Snipe in the build(2 Regulus with coordinated snipes is an excellent combo), but against skilled opponents the skill is not useful for much else than harassment.
In fortress games I will also go for range, to be able to take down the forts without sustaining damage.
-favor item: Heaven's Wrath(crazy exp bonus)
-store item: movement speed boots, (can't remember what they are called)
This build also allows for a much more versatile item selection. Crit gloves are included in my build in nearly every game, as well as one hp/mana boosting item. Depending on the game mode, map and opponents I will either go deep artifact(max dps) or max upgrades. With crit gloves, even without further damage increasing items, Regulus already makes an excellent chase hero, the addition of extra movement speed from the start just compliments this.
Skill order:
Mines 1
Maim 1
Enhanced attributes 1
Mines 2
Mark of the Betrayer 1
Enhanced Attributes 2
Maim 2
Mines 3
Enhanced Attributes 3
Mark of the Betrayer 2
From here on I skill depending on how the game is going. Generally I will max out Maim, mines and mark and also enhanced attributes.
The build itself seems to do quite well against most combinations, a high HP regulus seems to put many players off their game.
