Ring of Spell Turning
The Ring of Spell Turning is a magical item whose description in DMG p. 131 makes you want to give up and play 5th ed.
I have deciphered the mechanics and distilled them into a table
When the wearer of the ring is targeted by a spell, simply roll % dice. If the spell is a damage dealer, use the first column: it will tell you what % of the rolled dmg is reflected back at the caster.
If the spell is NOT a damage dealing spell (like charm person) and if there is a saving throw, use the second column. This is the saving throw bonus applied to BOTH the wearer of the ring and the enemy caster. Thus, on a 96, the enemy caster must ALSO save vs Charm Person at NO bonus or be charmed by her own spell.
If the spell does NOT allow a saving throw, consult the last two columns.
Gygax's love of one roll for many uses comes into play here. As you can see, the chart attempts to convert the complicated phrasing under "Note Regarding Ring of Spell Turning" on p. 131 into an easy cross-reference.
So in the case of Power Word Kill, for example, the ring wearer rolls % dice and a result of 34 comes up (exactly the same as in the DMG example). The saving throws generated by the ring are 18 and 14 respectively. The ring wearer needs an 18 or better or he is slain (15%). The enemy caster needs a 14 or better (35%), which doesn't exactly match the DMG's 30% because I'm rounding 66% up instead of down.
You are welcome to pore over the DMG version, compare and judge me a heretic if you like.