Here's how I've been playing Tahm Kench

Head support with a mid-or-lategame orientated ADC.

For masteries, use whatever mixture of defensive and utility points you'd grab for a tanky support; max out the three hp regeneration masteries.

For runes, 9 flat armor marks, 1 quint 1 flat hp regen seal, 2 quints 5 scaling hpregen seals 3 scaling hp seals 4 flat Mr glyphs 5 scaling Mr glyphs

Max greyhealth first, then Q, then devour.

Ridiculous amounts of sustain. :)