http://www.magelo.com/eq_spell_info.jsp?num=5105

What is the purpose of this?

http://lucy.allakhazam.com/spell.html?id=5105&source=Live

<table class="spellview" cellpadding="3" width="95%"><tbody><tr><td class="spelllabel" align="right">1: </td> <td>Mitigate Spell Damage by 20%, 500 total</td></tr> <tr><td class="spelllabel" align="right">9: </td> <td>Decrease Mana by 2 per tick</td></tr></tbody></table>

Okay, at least a benefit is showing there :)

If you're over the FT cap, does this have any negative effect? Or does it drop your MR by 2 regardless?

The FT cap applies to worn mana regen only.
This is a clicky (spell) effect, so it causes a loss of 2 mana/tick whenever it's in effect, regardless of your worn mana regen.

Thanks for clarifying that. It's the first "mixed blessing" type item I've seen.

Not in that you have to trade off weak stats (or focus etc) to gain some benefit you desire, but in that there is actually a negative effect to contend with. Big change in itemisation philosophy to have such items out there - I am surprised that given the existence of this one, there aren't more.

Or maybe there are, and I'm just too lowbie to see 'em :)

(final note - it makes sense now, given that it's a clicky. I mistakenly thought it was a worn effect, hence asking about "focus" in the title)

That'ld be good if you wanted to tank shei or exilied!