D&D 5E - Great Weapon Master
On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one weapon attack as a bonus action.
If you're looking for something that can help out a longsword wielder, this is it. If you're using a sword and shield, you're still able to use this part of the feat (although it's admittedly the weaker part of the feat). It also allows shenanigans like throwing your sword to kill a kobold at 60 feet (using it as an improvised thrown weapon: technically you'd kill it with a melee weapon so your bonus action attack would trigger), then dropping your shield, drawing your maul, and using the -5/+10 to splatter a kobold in front of you.
Before you make a melee weapon attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage.
This is by far the more powerful part of the feat (and one which has led to many arguments which I have no interest in rekindling). I think that if you allow this to be used on any Strength-based melee weapon (attack? All melee weapons are strength-based by default) it would take away the feat's niche of being mainly for 2-hand weapon users, so dual-wielders would be able to use its benefits as well as sword-and-board PCs.
The stereotypical longsword user can use a lot of feats to increase her combat capabilities: Notable examples are Heavy Armour Master, Mage Slayer, Sentinel, the aforementioned Shield Master... If the longsword user wants power (and insists on using a longsword) then they can always put their shield away and use it in two hands.
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