Tell that to the guy who killed me, every creep, and every boss in act one with a single flash of light.
That is easily accomplished using the Paladin's "Holy Shock" aura, which is a legitimate part of the game and certainly not a custom spell. In fact, any character in the game can obtain the Holy Shock aura simply by equipping the runeword "Dream" in a shield and/or helm - the Sorceress is particularly potent with this combination used in conjunction with her "Lightning Mastery" ability, she can actually raise her max melee damage over 50k with good gear. I have such a Sorceress (the Paladin skill Zeal is her primary attack) on closed B.net and I love her. :^)
There is also an exploit in the game which some cowardly players of Amazons use called "Farcast" which allows the Amazon to use her "Charged Strike" ability on players and monsters several screens away from where she actually is. While this is also not a custom spell, it certainly is a cheap exploit that no honest player would use. I bet what you saw was simply the Holy Shock aura, but this might also explain it.
There really are absolutely no custom spells on closed B.net. In fact I've never heard of one in any patch, ever, and I've been around a looooooooong time. There are some pretty cool spells on open B.net, for example characters who can use Diablo, Mephisto, and Baal's spells, and there are also some mods that really change things up (Median comes to mind) that can be played on open B.net. But as far as any of that is concerned, you have nothing to worry about on closed B.net. Nothing like that has ever happened and it probably never will.
The problems I have with closed B.net are: bots of all kinds (spam bots, baal bots, guber bots, all bots!), duping, d2jsp, and hateful people. That's why, when I choose to play Diablo II, I almost always do it in single player. I've now completed the game in single player (all three difficulties) in version 1.12 patch three times, with two paladins and a druid, and I'm currently in act 3 hell with a javazon. By solving and beating the challenges in the single player game, I have developed far greater skills and knowledge of the game than almost anyone I've ever met online. There was actually a time on closed B.net (back in version 1.08 and early 1.09) when players like me were actually something special, rare, and almost respected... now, thanks to duping, bots, and d2jsp, almost any ten year old kid can be "great" at the game in a week so nobody has any appreciation for the real blood n guts of the game.