How Frostfang works (incase you get autofilled)

Frostfang is the Tier 2 support item. This item has undergone a lot of changes, and autofilled supports taking the Spellthief's line of supp items often don't understand how to make it function well. Frostfang is absolutely essential as it doubles your stack use gold from Spellthiefs from 11 to 22 (per 10sec if you're dominating lane). Always upgrade this first, unless you're playing a mage and can afford Sorc Shoes first back. I see people buy Amplifying Tome+Boots over Frostfang+Boots which is such a big tradeoff in gold for such a marginal gain. +10 AP, -25% Mana regen -10% CDR and -11 gold per 10sec for Amp Tome route

Frostfang generates a stack every 10 seconds, and these stack up to 3 times. When you use a stack, you get 22 gold and deal 18 bonus magic damage. To use a stack you need to hit an enemy with a basic attack or ability

Frostfang also generates you 2 gold every 10 seconds. When you take a cs, that puts the passive gold generation AND stack generation on hold for 12 seconds

If you have 3 stacks and no way to use them in the near future, you can take cs with only a small tradeoff (of the passive gold, 2.4 gold per cs). This means it's okay to take one or two cs when you help shove->recall, or if you last hit while your adc is coming to lane. (Note: last hit means wait for the LETHAL tower/minion hit, and land your auto just before it lands, because otherwise you can push a little and deny your adc some cs)

If you're trying to keep frostfang stacks ticking though (which is how to finish your quest and get wards asap), it costs you 2.4 gold of generation and 26.4 gold from the stack you would proc. 28.8 gold per cs that your ADC could get isn't worth it ever. By doing this, you deny your adc and yourself gold. It's absolutely griefing to cop cs on purpose