In a game where I am paying a paladin/warlock, my DM was nice enough to let me swap Eldritch Blast for Greenflame Blade, with the warning that I'll lose it if warlocks do not end up getting this cantrip. Last night I got to test it out. For reference, I am playing a variant human paladin 1/ warlock 1 with Heavy Armour Master and a greatsword. During the session we faced a few encounters, though it only came up in two of them. The other player is a level 2 rogue.
1. 2 skeletons, 13 HP - open field, 40 foot separation
The enemies won initiative and moved towards us (DM had taken their bows away). I moved in, almost killed one, and lit the other one up for 3 fire damage using GFB. A round of misses later, I struck down the first one and again hit the other with GFB, reducing it to roughly half health. The rogue then finished it off at range.
Analysis: GFB provided a small amount of damage to a secondary target. After two rounds it would enough to defeat an enemy a single round sooner.
2. 12 zombies, 22 HP, DC 5+last damage taken to kill when reduced to 0HP - 3 foot doorway, ballbearings set in doorway
In this fight they could only get to me two at a time (one standing, one prone). The rogue stayed back and fired his bow while I held the doorway. This fight took somewhere close to 20 rounds before they all stopped moving, but on the whole they attacked two at a time, one would get slain and then another would advance. When reduced to 0HP, zombies fell prone before determining whether they passed their save or not. GFB was use primarily to force zombies at 1HP to remake the Con save, while regular weapon damage brought down healthy zombies.
Analysis: Unavoidable damage is great for situations where only a small amount of damage is needed to force a save with serious consequences. Almost every round I was forcing a zombie to make a DC 8 Con save or die. In a situation where there are casters supporting a more powerful creature, GFB could be used to force concentration checks.
Parting thoughts: GFB seems to be balanced for damage at low levels. The fire damage to a second target complements the original strike, though is strictly superior to merely attacking without extra attacks or features dependent on taking the Attack Action (monk's martial arts, dual wielding, shoves, grappling, etc.). The ability to force another creature to take damage may be slightly too powerful in situations where damage taken will cause a saving throw. Giving the secondary target a Dex save to avoid the damage might be useful for balancing this. Forced damage is previously unseen in cantrips, where a successful save results in no damage. As a potential counterpoint, the damage only occurs after a successful attack roll. Further testing is required to determine the severity of the problem.
I would be interested in reading other peoples' results using this cantrip, especially at higher levels.
