Singe - Deathly Spears inflict additional Fire damage. (34.7% from base damage, damage type fire)
Occult - Deathly Spears inflict additional Magic damage. (34.7% from base damage, damage type magic)
Jab - Spears strike repeatedly. (Adds additional attacks - at Combat Art level 1-15 a player will receive 2 waves, at Combat Art level 16 Deathly Spears will have 3 waves, at Combat Art level 42 a player will receive 4 waves of spikes and Jab continues to add extra attacks every 20 Combat Art levels after level 42.)
Bronze mod Batter is more defensive, as it will knock back enemies out of melee range. Note, however, that this will momentarily knock enemies safely out of range of subsequent waves of spikes, slightly decreasing the damage potential. Gore is the more offensive choice, for a Temple Guardian that expects to stay in the thick of battle and take damage.
The Jab Gold mod will increase its effectiveness as an AoE attack as well as a single target boss attack. The extra waves of Jab can continue to go off after the Temple Guardian has cast Deathly Spears, so he may use other Combat Arts while it is in effect. Eventually Jab will add so many waves of spikes, the Temple Guardian will be able to have multiple castings of Deathly Spears going simultaneously.
Note that, like all combat arts in the Devout Guardian aspect, Deathly Spears uses weapon attack speed. Even though the weapon itself isn't used for this CA, higher weapon speed = faster execution of Deathly Spears. Therefore, taking a weapon lore skill is important to maximize the potential of Deathly Spears.
Pros and Cons
Can produce a high amount of damage
Deathly Spears is considered an area of attack spell and can be used against "cloaked" mobs/players
Great for both single target and multiple target encounters
Must be at high levels to see good results
The Jab mod is almost a "must-have" for this Combat Art which is the final gold mod, this can take time to reach since there are so many other important mods to take for the Temple guardian within the Devout Guardian aspect